-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. There are
separate text files for the pcregrep and pcretest commands.
-----------------------------------------------------------------------------
PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
INTRODUCTION
The PCRE library is a set of functions that implement regular expres-
sion pattern matching using the same syntax and semantics as Perl, with
just a few differences. Certain features that appeared in Python and
PCRE before they appeared in Perl are also available using the Python
syntax. There is also some support for certain .NET and Oniguruma syn-
tax items, and there is an option for requesting some minor changes
that give better JavaScript compatibility.
The current implementation of PCRE (release 7.x) corresponds approxi-
mately with Perl 5.10, including support for UTF-8 encoded strings and
Unicode general category properties. However, UTF-8 and Unicode support
has to be explicitly enabled; it is not the default. The Unicode tables
correspond to Unicode release 5.0.0.
In addition to the Perl-compatible matching function, PCRE contains an
alternative matching function that matches the same compiled patterns
in a different way. In certain circumstances, the alternative function
has some advantages. For a discussion of the two matching algorithms,
see the pcrematching page.
PCRE is written in C and released as a C library. A number of people
have written wrappers and interfaces of various kinds. In particular,
Google Inc. have provided a comprehensive C++ wrapper. This is now
included as part of the PCRE distribution. The pcrecpp page has details
of this interface. Other people's contributions can be found in the
Contrib directory at the primary FTP site, which is:
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
Details of exactly which Perl regular expression features are and are
not supported by PCRE are given in separate documents. See the pcrepat-
tern and pcrecompat pages. There is a syntax summary in the pcresyntax
page.
Some features of PCRE can be included, excluded, or changed when the
library is built. The pcre_config() function makes it possible for a
client to discover which features are available. The features them-
selves are described in the pcrebuild page. Documentation about build-
ing PCRE for various operating systems can be found in the README file
in the source distribution.
The library contains a number of undocumented internal functions and
data tables that are used by more than one of the exported external
functions, but which are not intended for use by external callers.
Their names all begin with "_pcre_", which hopefully will not provoke
any name clashes. In some environments, it is possible to control which
external symbols are exported when a shared library is built, and in
these cases the undocumented symbols are not exported.
USER DOCUMENTATION
The user documentation for PCRE comprises a number of different sec-
tions. In the "man" format, each of these is a separate "man page". In
the HTML format, each is a separate page, linked from the index page.
In the plain text format, all the sections are concatenated, for ease
of searching. The sections are as follows:
pcre this document
pcre-config show PCRE installation configuration information
pcreapi details of PCRE's native C API
pcrebuild options for building PCRE
pcrecallout details of the callout feature
pcrecompat discussion of Perl compatibility
pcrecpp details of the C++ wrapper
pcregrep description of the pcregrep command
pcrematching discussion of the two matching algorithms
pcrepartial details of the partial matching facility
pcrepattern syntax and semantics of supported
regular expressions
pcresyntax quick syntax reference
pcreperform discussion of performance issues
pcreposix the POSIX-compatible C API
pcreprecompile details of saving and re-using precompiled patterns
pcresample discussion of the sample program
pcrestack discussion of stack usage
pcretest description of the pcretest testing command
In addition, in the "man" and HTML formats, there is a short page for
each C library function, listing its arguments and results.
LIMITATIONS
There are some size limitations in PCRE but it is hoped that they will
never in practice be relevant.
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
is compiled with the default internal linkage size of 2. If you want to
process regular expressions that are truly enormous, you can compile
PCRE with an internal linkage size of 3 or 4 (see the README file in
the source distribution and the pcrebuild documentation for details).
In these cases the limit is substantially larger. However, the speed
of execution is slower.
All values in repeating quantifiers must be less than 65536.
There is no limit to the number of parenthesized subpatterns, but there
can be no more than 65535 capturing subpatterns.
The maximum length of name for a named subpattern is 32 characters, and
the maximum number of named subpatterns is 10000.
The maximum length of a subject string is the largest positive number
that an integer variable can hold. However, when using the traditional
matching function, PCRE uses recursion to handle subpatterns and indef-
inite repetition. This means that the available stack space may limit
the size of a subject string that can be processed by certain patterns.
For a discussion of stack issues, see the pcrestack documentation.
UTF-8 AND UNICODE PROPERTY SUPPORT
From release 3.3, PCRE has had some support for character strings
encoded in the UTF-8 format. For release 4.0 this was greatly extended
to cover most common requirements, and in release 5.0 additional sup-
port for Unicode general category properties was added.
In order process UTF-8 strings, you must build PCRE to include UTF-8
support in the code, and, in addition, you must call pcre_compile()
with the PCRE_UTF8 option flag. When you do this, both the pattern and
any subject strings that are matched against it are treated as UTF-8
strings instead of just strings of bytes.
If you compile PCRE with UTF-8 support, but do not use it at run time,
the library will be a bit bigger, but the additional run time overhead
is limited to testing the PCRE_UTF8 flag occasionally, so should not be
very big.
If PCRE is built with Unicode character property support (which implies
UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
ported. The available properties that can be tested are limited to the
general category properties such as Lu for an upper case letter or Nd
for a decimal number, the Unicode script names such as Arabic or Han,
and the derived properties Any and L&. A full list is given in the
pcrepattern documentation. Only the short names for properties are sup-
ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
ter}, is not supported. Furthermore, in Perl, many properties may
optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
does not support this.
Validity of UTF-8 strings
When you set the PCRE_UTF8 flag, the strings passed as patterns and
subjects are (by default) checked for validity on entry to the relevant
functions. From release 7.3 of PCRE, the check is according the rules
of RFC 3629, which are themselves derived from the Unicode specifica-
tion. Earlier releases of PCRE followed the rules of RFC 2279, which
allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
check allows only values in the range U+0 to U+10FFFF, excluding U+D800
to U+DFFF.
The excluded code points are the "Low Surrogate Area" of Unicode, of
which the Unicode Standard says this: "The Low Surrogate Area does not
contain any character assignments, consequently no character code
charts or namelists are provided for this area. Surrogates are reserved
for use with UTF-16 and then must be used in pairs." The code points
that are encoded by UTF-16 pairs are available as independent code
points in the UTF-8 encoding. (In other words, the whole surrogate
thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
If an invalid UTF-8 string is passed to PCRE, an error return
(PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
that your strings are valid, and therefore want to skip these checks in
order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
compile time or at run time, PCRE assumes that the pattern or subject
it is given (respectively) contains only valid UTF-8 codes. In this
case, it does not diagnose an invalid UTF-8 string.
If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
what happens depends on why the string is invalid. If the string con-
forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
string of characters in the range 0 to 0x7FFFFFFF. In other words,
apart from the initial validity test, PCRE (when in UTF-8 mode) handles
strings according to the more liberal rules of RFC 2279. However, if
the string does not even conform to RFC 2279, the result is undefined.
Your program may crash.
If you want to process strings of values in the full range 0 to
0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
this situation, you will have to apply your own validity check.
General comments about UTF-8 mode
1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
two-byte UTF-8 character if the value is greater than 127.
2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
characters for values greater than \177.
3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
vidual bytes, for example: \x{100}{3}.
4. The dot metacharacter matches one UTF-8 character instead of a sin-
gle byte.
5. The escape sequence \C can be used to match a single byte in UTF-8
mode, but its use can lead to some strange effects. This facility is
not available in the alternative matching function, pcre_dfa_exec().
6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but the characters that PCRE recog-
nizes as digits, spaces, or word characters remain the same set as
before, all with values less than 256. This remains true even when PCRE
includes Unicode property support, because to do otherwise would slow
down PCRE in many common cases. If you really want to test for a wider
sense of, say, "digit", you must use Unicode property tests such as
\p{Nd}.
7. Similarly, characters that match the POSIX named character classes
are all low-valued characters.
8. However, the Perl 5.10 horizontal and vertical whitespace matching
escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
acters.
9. Case-insensitive matching applies only to characters whose values
are less than 128, unless PCRE is built with Unicode property support.
Even when Unicode property support is available, PCRE still uses its
own character tables when checking the case of low-valued characters,
so as not to degrade performance. The Unicode property information is
used only for characters with higher values. Even when Unicode property
support is available, PCRE supports case-insensitive matching only when
there is a one-to-one mapping between a letter's cases. There are a
small number of many-to-one mappings in Unicode; these are not sup-
ported by PCRE.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
Putting an actual email address here seems to have been a spam magnet,
so I've taken it away. If you want to email me, use my two initials,
followed by the two digits 10, at the domain cam.ac.uk.
REVISION
Last updated: 12 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCREBUILD(3) PCREBUILD(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE BUILD-TIME OPTIONS
This document describes the optional features of PCRE that can be
selected when the library is compiled. It assumes use of the configure
script, where the optional features are selected or deselected by pro-
viding options to configure before running the make command. However,
the same options can be selected in both Unix-like and non-Unix-like
environments using the GUI facility of CMakeSetup if you are using
CMake instead of configure to build PCRE.
The complete list of options for configure (which includes the standard
ones such as the selection of the installation directory) can be
obtained by running
./configure --help
The following sections include descriptions of options whose names
begin with --enable or --disable. These settings specify changes to the
defaults for the configure command. Because of the way that configure
works, --enable and --disable always come in pairs, so the complemen-
tary option always exists as well, but as it specifies the default, it
is not described.
C++ SUPPORT
By default, the configure script will search for a C++ compiler and C++
header files. If it finds them, it automatically builds the C++ wrapper
library for PCRE. You can disable this by adding
--disable-cpp
to the configure command.
UTF-8 SUPPORT
To build PCRE with support for UTF-8 character strings, add
--enable-utf8
to the configure command. Of itself, this does not make PCRE treat
strings as UTF-8. As well as compiling PCRE with this option, you also
have have to set the PCRE_UTF8 option when you call the pcre_compile()
function.
UNICODE CHARACTER PROPERTY SUPPORT
UTF-8 support allows PCRE to process character values greater than 255
in the strings that it handles. On its own, however, it does not pro-
vide any facilities for accessing the properties of such characters. If
you want to be able to use the pattern escapes \P, \p, and \X, which
refer to Unicode character properties, you must add
--enable-unicode-properties
to the configure command. This implies UTF-8 support, even if you have
not explicitly requested it.
Including Unicode property support adds around 30K of tables to the
PCRE library. Only the general category properties such as Lu and Nd
are supported. Details are given in the pcrepattern documentation.
CODE VALUE OF NEWLINE
By default, PCRE interprets character 10 (linefeed, LF) as indicating
the end of a line. This is the normal newline character on Unix-like
systems. You can compile PCRE to use character 13 (carriage return, CR)
instead, by adding
--enable-newline-is-cr
to the configure command. There is also a --enable-newline-is-lf
option, which explicitly specifies linefeed as the newline character.
Alternatively, you can specify that line endings are to be indicated by
the two character sequence CRLF. If you want this, add
--enable-newline-is-crlf
to the configure command. There is a fourth option, specified by
--enable-newline-is-anycrlf
which causes PCRE to recognize any of the three sequences CR, LF, or
CRLF as indicating a line ending. Finally, a fifth option, specified by
--enable-newline-is-any
causes PCRE to recognize any Unicode newline sequence.
Whatever line ending convention is selected when PCRE is built can be
overridden when the library functions are called. At build time it is
conventional to use the standard for your operating system.
WHAT \R MATCHES
By default, the sequence \R in a pattern matches any Unicode newline
sequence, whatever has been selected as the line ending sequence. If
you specify
--enable-bsr-anycrlf
the default is changed so that \R matches only CR, LF, or CRLF. What-
ever is selected when PCRE is built can be overridden when the library
functions are called.
BUILDING SHARED AND STATIC LIBRARIES
The PCRE building process uses libtool to build both shared and static
Unix libraries by default. You can suppress one of these by adding one
of
--disable-shared
--disable-static
to the configure command, as required.
POSIX MALLOC USAGE
When PCRE is called through the POSIX interface (see the pcreposix doc-
umentation), additional working storage is required for holding the
pointers to capturing substrings, because PCRE requires three integers
per substring, whereas the POSIX interface provides only two. If the
number of expected substrings is small, the wrapper function uses space
on the stack, because this is faster than using malloc() for each call.
The default threshold above which the stack is no longer used is 10; it
can be changed by adding a setting such as
--with-posix-malloc-threshold=20
to the configure command.
HANDLING VERY LARGE PATTERNS
Within a compiled pattern, offset values are used to point from one
part to another (for example, from an opening parenthesis to an alter-
nation metacharacter). By default, two-byte values are used for these
offsets, leading to a maximum size for a compiled pattern of around
64K. This is sufficient to handle all but the most gigantic patterns.
Nevertheless, some people do want to process enormous patterns, so it
is possible to compile PCRE to use three-byte or four-byte offsets by
adding a setting such as
--with-link-size=3
to the configure command. The value given must be 2, 3, or 4. Using
longer offsets slows down the operation of PCRE because it has to load
additional bytes when handling them.
AVOIDING EXCESSIVE STACK USAGE
When matching with the pcre_exec() function, PCRE implements backtrack-
ing by making recursive calls to an internal function called match().
In environments where the size of the stack is limited, this can se-
verely limit PCRE's operation. (The Unix environment does not usually
suffer from this problem, but it may sometimes be necessary to increase
the maximum stack size. There is a discussion in the pcrestack docu-
mentation.) An alternative approach to recursion that uses memory from
the heap to remember data, instead of using recursive function calls,
has been implemented to work round the problem of limited stack size.
If you want to build a version of PCRE that works this way, add
--disable-stack-for-recursion
to the configure command. With this configuration, PCRE will use the
pcre_stack_malloc and pcre_stack_free variables to call memory manage-
ment functions. By default these point to malloc() and free(), but you
can replace the pointers so that your own functions are used.
Separate functions are provided rather than using pcre_malloc and
pcre_free because the usage is very predictable: the block sizes
requested are always the same, and the blocks are always freed in
reverse order. A calling program might be able to implement optimized
functions that perform better than malloc() and free(). PCRE runs
noticeably more slowly when built in this way. This option affects only
the pcre_exec() function; it is not relevant for the the
pcre_dfa_exec() function.
LIMITING PCRE RESOURCE USAGE
Internally, PCRE has a function called match(), which it calls repeat-
edly (sometimes recursively) when matching a pattern with the
pcre_exec() function. By controlling the maximum number of times this
function may be called during a single matching operation, a limit can
be placed on the resources used by a single call to pcre_exec(). The
limit can be changed at run time, as described in the pcreapi documen-
tation. The default is 10 million, but this can be changed by adding a
setting such as
--with-match-limit=500000
to the configure command. This setting has no effect on the
pcre_dfa_exec() matching function.
In some environments it is desirable to limit the depth of recursive
calls of match() more strictly than the total number of calls, in order
to restrict the maximum amount of stack (or heap, if --disable-stack-
for-recursion is specified) that is used. A second limit controls this;
it defaults to the value that is set for --with-match-limit, which
imposes no additional constraints. However, you can set a lower limit
by adding, for example,
--with-match-limit-recursion=10000
to the configure command. This value can also be overridden at run
time.
CREATING CHARACTER TABLES AT BUILD TIME
PCRE uses fixed tables for processing characters whose code values are
less than 256. By default, PCRE is built with a set of tables that are
distributed in the file pcre_chartables.c.dist. These tables are for
ASCII codes only. If you add
--enable-rebuild-chartables
to the configure command, the distributed tables are no longer used.
Instead, a program called dftables is compiled and run. This outputs
the source for new set of tables, created in the default locale of your
C runtime system. (This method of replacing the tables does not work if
you are cross compiling, because dftables is run on the local host. If
you need to create alternative tables when cross compiling, you will
have to do so "by hand".)
USING EBCDIC CODE
PCRE assumes by default that it will run in an environment where the
character code is ASCII (or Unicode, which is a superset of ASCII).
This is the case for most computer operating systems. PCRE can, how-
ever, be compiled to run in an EBCDIC environment by adding
--enable-ebcdic
to the configure command. This setting implies --enable-rebuild-charta-
bles. You should only use it if you know that you are in an EBCDIC
environment (for example, an IBM mainframe operating system).
PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
By default, pcregrep reads all files as plain text. You can build it so
that it recognizes files whose names end in .gz or .bz2, and reads them
with libz or libbz2, respectively, by adding one or both of
--enable-pcregrep-libz
--enable-pcregrep-libbz2
to the configure command. These options naturally require that the rel-
evant libraries are installed on your system. Configuration will fail
if they are not.
PCRETEST OPTION FOR LIBREADLINE SUPPORT
If you add
--enable-pcretest-libreadline
to the configure command, pcretest is linked with the libreadline
library, and when its input is from a terminal, it reads it using the
readline() function. This provides line-editing and history facilities.
Note that libreadline is GPL-licenced, so if you distribute a binary of
pcretest linked in this way, there may be licensing issues.
Setting this option causes the -lreadline option to be added to the
pcretest build. In many operating environments with a sytem-installed
libreadline this is sufficient. However, in some environments (e.g. if
an unmodified distribution version of readline is in use), some extra
configuration may be necessary. The INSTALL file for libreadline says
this:
"Readline uses the termcap functions, but does not link with the
termcap or curses library itself, allowing applications which link
with readline the to choose an appropriate library."
If your environment has not been set up so that an appropriate library
is automatically included, you may need to add something like
LIBS="-ncurses"
immediately before the configure command.
SEE ALSO
pcreapi(3), pcre_config(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 13 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCREMATCHING(3) PCREMATCHING(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE MATCHING ALGORITHMS
This document describes the two different algorithms that are available
in PCRE for matching a compiled regular expression against a given sub-
ject string. The "standard" algorithm is the one provided by the
pcre_exec() function. This works in the same was as Perl's matching
function, and provides a Perl-compatible matching operation.
An alternative algorithm is provided by the pcre_dfa_exec() function;
this operates in a different way, and is not Perl-compatible. It has
advantages and disadvantages compared with the standard algorithm, and
these are described below.
When there is only one possible way in which a given subject string can
match a pattern, the two algorithms give the same answer. A difference
arises, however, when there are multiple possibilities. For example, if
the pattern
^<.*>
is matched against the string
<something> <something else> <something further>
there are three possible answers. The standard algorithm finds only one
of them, whereas the alternative algorithm finds all three.
REGULAR EXPRESSIONS AS TREES
The set of strings that are matched by a regular expression can be rep-
resented as a tree structure. An unlimited repetition in the pattern
makes the tree of infinite size, but it is still a tree. Matching the
pattern to a given subject string (from a given starting point) can be
thought of as a search of the tree. There are two ways to search a
tree: depth-first and breadth-first, and these correspond to the two
matching algorithms provided by PCRE.
THE STANDARD MATCHING ALGORITHM
In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
sions", the standard algorithm is an "NFA algorithm". It conducts a
depth-first search of the pattern tree. That is, it proceeds along a
single path through the tree, checking that the subject matches what is
required. When there is a mismatch, the algorithm tries any alterna-
tives at the current point, and if they all fail, it backs up to the
previous branch point in the tree, and tries the next alternative
branch at that level. This often involves backing up (moving to the
left) in the subject string as well. The order in which repetition
branches are tried is controlled by the greedy or ungreedy nature of
the quantifier.
If a leaf node is reached, a matching string has been found, and at
that point the algorithm stops. Thus, if there is more than one possi-
ble match, this algorithm returns the first one that it finds. Whether
this is the shortest, the longest, or some intermediate length depends
on the way the greedy and ungreedy repetition quantifiers are specified
in the pattern.
Because it ends up with a single path through the tree, it is rela-
tively straightforward for this algorithm to keep track of the sub-
strings that are matched by portions of the pattern in parentheses.
This provides support for capturing parentheses and back references.
THE ALTERNATIVE MATCHING ALGORITHM
This algorithm conducts a breadth-first search of the tree. Starting
from the first matching point in the subject, it scans the subject
string from left to right, once, character by character, and as it does
this, it remembers all the paths through the tree that represent valid
matches. In Friedl's terminology, this is a kind of "DFA algorithm",
though it is not implemented as a traditional finite state machine (it
keeps multiple states active simultaneously).
The scan continues until either the end of the subject is reached, or
there are no more unterminated paths. At this point, terminated paths
represent the different matching possibilities (if there are none, the
match has failed). Thus, if there is more than one possible match,
this algorithm finds all of them, and in particular, it finds the long-
est. In PCRE, there is an option to stop the algorithm after the first
match (which is necessarily the shortest) has been found.
Note that all the matches that are found start at the same point in the
subject. If the pattern
cat(er(pillar)?)
is matched against the string "the caterpillar catchment", the result
will be the three strings "cat", "cater", and "caterpillar" that start
at the fourth character of the subject. The algorithm does not automat-
ically move on to find matches that start at later positions.
There are a number of features of PCRE regular expressions that are not
supported by the alternative matching algorithm. They are as follows:
1. Because the algorithm finds all possible matches, the greedy or
ungreedy nature of repetition quantifiers is not relevant. Greedy and
ungreedy quantifiers are treated in exactly the same way. However, pos-
sessive quantifiers can make a difference when what follows could also
match what is quantified, for example in a pattern like this:
^a++\w!
This pattern matches "aaab!" but not "aaa!", which would be matched by
a non-possessive quantifier. Similarly, if an atomic group is present,
it is matched as if it were a standalone pattern at the current point,
and the longest match is then "locked in" for the rest of the overall
pattern.
2. When dealing with multiple paths through the tree simultaneously, it
is not straightforward to keep track of captured substrings for the
different matching possibilities, and PCRE's implementation of this
algorithm does not attempt to do this. This means that no captured sub-
strings are available.
3. Because no substrings are captured, back references within the pat-
tern are not supported, and cause errors if encountered.
4. For the same reason, conditional expressions that use a backrefer-
ence as the condition or test for a specific group recursion are not
supported.
5. Because many paths through the tree may be active, the \K escape
sequence, which resets the start of the match when encountered (but may
be on some paths and not on others), is not supported. It causes an
error if encountered.
6. Callouts are supported, but the value of the capture_top field is
always 1, and the value of the capture_last field is always -1.
7. The \C escape sequence, which (in the standard algorithm) matches a
single byte, even in UTF-8 mode, is not supported because the alterna-
tive algorithm moves through the subject string one character at a
time, for all active paths through the tree.
8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
are not supported. (*FAIL) is supported, and behaves like a failing
negative assertion.
ADVANTAGES OF THE ALTERNATIVE ALGORITHM
Using the alternative matching algorithm provides the following advan-
tages:
1. All possible matches (at a single point in the subject) are automat-
ically found, and in particular, the longest match is found. To find
more than one match using the standard algorithm, you have to do kludgy
things with callouts.
2. There is much better support for partial matching. The restrictions
on the content of the pattern that apply when using the standard algo-
rithm for partial matching do not apply to the alternative algorithm.
For non-anchored patterns, the starting position of a partial match is
available.
3. Because the alternative algorithm scans the subject string just
once, and never needs to backtrack, it is possible to pass very long
subject strings to the matching function in several pieces, checking
for partial matching each time.
DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
The alternative algorithm suffers from a number of disadvantages:
1. It is substantially slower than the standard algorithm. This is
partly because it has to search for all possible matches, but is also
because it is less susceptible to optimization.
2. Capturing parentheses and back references are not supported.
3. Although atomic groups are supported, their use does not provide the
performance advantage that it does for the standard algorithm.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 19 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCREAPI(3) PCREAPI(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE NATIVE API
#include <pcre.h>
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre_extra *pcre_study(const pcre *code, int options,
const char **errptr);
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
void pcre_free_substring(const char *stringptr);
void pcre_free_substring_list(const char **stringptr);
const unsigned char *pcre_maketables(void);
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
int pcre_refcount(pcre *code, int adjust);
int pcre_config(int what, void *where);
char *pcre_version(void);
void *(*pcre_malloc)(size_t);
void (*pcre_free)(void *);
void *(*pcre_stack_malloc)(size_t);
void (*pcre_stack_free)(void *);
int (*pcre_callout)(pcre_callout_block *);
PCRE API OVERVIEW
PCRE has its own native API, which is described in this document. There
are also some wrapper functions that correspond to the POSIX regular
expression API. These are described in the pcreposix documentation.
Both of these APIs define a set of C function calls. A C++ wrapper is
distributed with PCRE. It is documented in the pcrecpp page.
The native API C function prototypes are defined in the header file
pcre.h, and on Unix systems the library itself is called libpcre. It
can normally be accessed by adding -lpcre to the command for linking an
application that uses PCRE. The header file defines the macros
PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
bers for the library. Applications can use these to include support
for different releases of PCRE.
The functions pcre_compile(), pcre_compile2(), pcre_study(), and
pcre_exec() are used for compiling and matching regular expressions in
a Perl-compatible manner. A sample program that demonstrates the sim-
plest way of using them is provided in the file called pcredemo.c in
the source distribution. The pcresample documentation describes how to
compile and run it.
A second matching function, pcre_dfa_exec(), which is not Perl-compati-
ble, is also provided. This uses a different algorithm for the match-
ing. The alternative algorithm finds all possible matches (at a given
point in the subject), and scans the subject just once. However, this
algorithm does not return captured substrings. A description of the two
matching algorithms and their advantages and disadvantages is given in
the pcrematching documentation.
In addition to the main compiling and matching functions, there are
convenience functions for extracting captured substrings from a subject
string that is matched by pcre_exec(). They are:
pcre_copy_substring()
pcre_copy_named_substring()
pcre_get_substring()
pcre_get_named_substring()
pcre_get_substring_list()
pcre_get_stringnumber()
pcre_get_stringtable_entries()
pcre_free_substring() and pcre_free_substring_list() are also provided,
to free the memory used for extracted strings.
The function pcre_maketables() is used to build a set of character
tables in the current locale for passing to pcre_compile(),
pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
provided for specialist use. Most commonly, no special tables are
passed, in which case internal tables that are generated when PCRE is
built are used.
The function pcre_fullinfo() is used to find out information about a
compiled pattern; pcre_info() is an obsolete version that returns only
some of the available information, but is retained for backwards com-
patibility. The function pcre_version() returns a pointer to a string
containing the version of PCRE and its date of release.
The function pcre_refcount() maintains a reference count in a data
block containing a compiled pattern. This is provided for the benefit
of object-oriented applications.
The global variables pcre_malloc and pcre_free initially contain the
entry points of the standard malloc() and free() functions, respec-
tively. PCRE calls the memory management functions via these variables,
so a calling program can replace them if it wishes to intercept the
calls. This should be done before calling any PCRE functions.
The global variables pcre_stack_malloc and pcre_stack_free are also
indirections to memory management functions. These special functions
are used only when PCRE is compiled to use the heap for remembering
data, instead of recursive function calls, when running the pcre_exec()
function. See the pcrebuild documentation for details of how to do
this. It is a non-standard way of building PCRE, for use in environ-
ments that have limited stacks. Because of the greater use of memory
management, it runs more slowly. Separate functions are provided so
that special-purpose external code can be used for this case. When
used, these functions are always called in a stack-like manner (last
obtained, first freed), and always for memory blocks of the same size.
There is a discussion about PCRE's stack usage in the pcrestack docu-
mentation.
The global variable pcre_callout initially contains NULL. It can be set
by the caller to a "callout" function, which PCRE will then call at
specified points during a matching operation. Details are given in the
pcrecallout documentation.
NEWLINES
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line-
feed) character, the two-character sequence CRLF, any of the three pre-
ceding, or any Unicode newline sequence. The Unicode newline sequences
are the three just mentioned, plus the single characters VT (vertical
tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
separator, U+2028), and PS (paragraph separator, U+2029).
Each of the first three conventions is used by at least one operating
system as its standard newline sequence. When PCRE is built, a default
can be specified. The default default is LF, which is the Unix stan-
dard. When PCRE is run, the default can be overridden, either when a
pattern is compiled, or when it is matched.
At compile time, the newline convention can be specified by the options
argument of pcre_compile(), or it can be specified by special text at
the start of the pattern itself; this overrides any other settings. See
the pcrepattern page for details of the special character sequences.
In the PCRE documentation the word "newline" is used to mean "the char-
acter or pair of characters that indicate a line break". The choice of
newline convention affects the handling of the dot, circumflex, and
dollar metacharacters, the handling of #-comments in /x mode, and, when
CRLF is a recognized line ending sequence, the match position advance-
ment for a non-anchored pattern. There is more detail about this in the
section on pcre_exec() options below.
The choice of newline convention does not affect the interpretation of
the \n or \r escape sequences, nor does it affect what \R matches,
which is controlled in a similar way, but by separate options.
MULTITHREADING
The PCRE functions can be used in multi-threading applications, with
the proviso that the memory management functions pointed to by
pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
callout function pointed to by pcre_callout, are shared by all threads.
The compiled form of a regular expression is not altered during match-
ing, so the same compiled pattern can safely be used by several threads
at once.
SAVING PRECOMPILED PATTERNS FOR LATER USE
The compiled form of a regular expression can be saved and re-used at a
later time, possibly by a different program, and even on a host other
than the one on which it was compiled. Details are given in the
pcreprecompile documentation. However, compiling a regular expression
with one version of PCRE for use with a different version is not guar-
anteed to work and may cause crashes.
CHECKING BUILD-TIME OPTIONS
int pcre_config(int what, void *where);
The function pcre_config() makes it possible for a PCRE client to dis-
cover which optional features have been compiled into the PCRE library.
The pcrebuild documentation has more details about these optional fea-
tures.
The first argument for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to a variable
into which the information is placed. The following information is
available:
PCRE_CONFIG_UTF8
The output is an integer that is set to one if UTF-8 support is avail-
able; otherwise it is set to zero.
PCRE_CONFIG_UNICODE_PROPERTIES
The output is an integer that is set to one if support for Unicode
character properties is available; otherwise it is set to zero.
PCRE_CONFIG_NEWLINE
The output is an integer whose value specifies the default character
sequence that is recognized as meaning "newline". The four values that
are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
and -1 for ANY. The default should normally be the standard sequence
for your operating system.
PCRE_CONFIG_BSR
The output is an integer whose value indicates what character sequences
the \R escape sequence matches by default. A value of 0 means that \R
matches any Unicode line ending sequence; a value of 1 means that \R
matches only CR, LF, or CRLF. The default can be overridden when a pat-
tern is compiled or matched.
PCRE_CONFIG_LINK_SIZE
The output is an integer that contains the number of bytes used for
internal linkage in compiled regular expressions. The value is 2, 3, or
4. Larger values allow larger regular expressions to be compiled, at
the expense of slower matching. The default value of 2 is sufficient
for all but the most massive patterns, since it allows the compiled
pattern to be up to 64K in size.
PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
The output is an integer that contains the threshold above which the
POSIX interface uses malloc() for output vectors. Further details are
given in the pcreposix documentation.
PCRE_CONFIG_MATCH_LIMIT
The output is an integer that gives the default limit for the number of
internal matching function calls in a pcre_exec() execution. Further
details are given with pcre_exec() below.
PCRE_CONFIG_MATCH_LIMIT_RECURSION
The output is an integer that gives the default limit for the depth of
recursion when calling the internal matching function in a pcre_exec()
execution. Further details are given with pcre_exec() below.
PCRE_CONFIG_STACKRECURSE
The output is an integer that is set to one if internal recursion when
running pcre_exec() is implemented by recursive function calls that use
the stack to remember their state. This is the usual way that PCRE is
compiled. The output is zero if PCRE was compiled to use blocks of data
on the heap instead of recursive function calls. In this case,
pcre_stack_malloc and pcre_stack_free are called to manage memory
blocks on the heap, thus avoiding the use of the stack.
COMPILING A PATTERN
pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
Either of the functions pcre_compile() or pcre_compile2() can be called
to compile a pattern into an internal form. The only difference between
the two interfaces is that pcre_compile2() has an additional argument,
errorcodeptr, via which a numerical error code can be returned.
The pattern is a C string terminated by a binary zero, and is passed in
the pattern argument. A pointer to a single block of memory that is
obtained via pcre_malloc is returned. This contains the compiled code
and related data. The pcre type is defined for the returned block; this
is a typedef for a structure whose contents are not externally defined.
It is up to the caller to free the memory (via pcre_free) when it is no
longer required.
Although the compiled code of a PCRE regex is relocatable, that is, it
does not depend on memory location, the complete pcre data block is not
fully relocatable, because it may contain a copy of the tableptr argu-
ment, which is an address (see below).
The options argument contains various bit settings that affect the com-
pilation. It should be zero if no options are required. The available
options are described below. Some of them, in particular, those that
are compatible with Perl, can also be set and unset from within the
pattern (see the detailed description in the pcrepattern documenta-
tion). For these options, the contents of the options argument speci-
fies their initial settings at the start of compilation and execution.
The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
of matching as well as at compile time.
If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
if compilation of a pattern fails, pcre_compile() returns NULL, and
sets the variable pointed to by errptr to point to a textual error mes-
sage. This is a static string that is part of the library. You must not
try to free it. The offset from the start of the pattern to the charac-
ter where the error was discovered is placed in the variable pointed to
by erroffset, which must not be NULL. If it is, an immediate error is
given.
If pcre_compile2() is used instead of pcre_compile(), and the error-
codeptr argument is not NULL, a non-zero error code number is returned
via this argument in the event of an error. This is in addition to the
textual error message. Error codes and messages are listed below.
If the final argument, tableptr, is NULL, PCRE uses a default set of
character tables that are built when PCRE is compiled, using the
default C locale. Otherwise, tableptr must be an address that is the
result of a call to pcre_maketables(). This value is stored with the
compiled pattern, and used again by pcre_exec(), unless another table
pointer is passed to it. For more discussion, see the section on locale
support below.
This code fragment shows a typical straightforward call to pcre_com-
pile():
pcre *re;
const char *error;
int erroffset;
re = pcre_compile(
"^A.*Z", /* the pattern */
0, /* default options */
&error;, /* for error message */
&erroffset;, /* for error offset */
NULL); /* use default character tables */
The following names for option bits are defined in the pcre.h header
file:
PCRE_ANCHORED
If this bit is set, the pattern is forced to be "anchored", that is, it
is constrained to match only at the first matching point in the string
that is being searched (the "subject string"). This effect can also be
achieved by appropriate constructs in the pattern itself, which is the
only way to do it in Perl.
PCRE_AUTO_CALLOUT
If this bit is set, pcre_compile() automatically inserts callout items,
all with number 255, before each pattern item. For discussion of the
callout facility, see the pcrecallout documentation.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R escape
sequence matches. The choice is either to match only CR, LF, or CRLF,
or to match any Unicode newline sequence. The default is specified when
PCRE is built. It can be overridden from within the pattern, or by set-
ting an option when a compiled pattern is matched.
PCRE_CASELESS
If this bit is set, letters in the pattern match both upper and lower
case letters. It is equivalent to Perl's /i option, and it can be
changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
always understands the concept of case for characters whose values are
less than 128, so caseless matching is always possible. For characters
with higher values, the concept of case is supported if PCRE is com-
piled with Unicode property support, but not otherwise. If you want to
use caseless matching for characters 128 and above, you must ensure
that PCRE is compiled with Unicode property support as well as with
UTF-8 support.
PCRE_DOLLAR_ENDONLY
If this bit is set, a dollar metacharacter in the pattern matches only
at the end of the subject string. Without this option, a dollar also
matches immediately before a newline at the end of the string (but not
before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
if PCRE_MULTILINE is set. There is no equivalent to this option in
Perl, and no way to set it within a pattern.
PCRE_DOTALL
If this bit is set, a dot metacharater in the pattern matches all char-
acters, including those that indicate newline. Without it, a dot does
not match when the current position is at a newline. This option is
equivalent to Perl's /s option, and it can be changed within a pattern
by a (?s) option setting. A negative class such as [^a] always matches
newline characters, independent of the setting of this option.
PCRE_DUPNAMES
If this bit is set, names used to identify capturing subpatterns need
not be unique. This can be helpful for certain types of pattern when it
is known that only one instance of the named subpattern can ever be
matched. There are more details of named subpatterns below; see also
the pcrepattern documentation.
PCRE_EXTENDED
If this bit is set, whitespace data characters in the pattern are
totally ignored except when escaped or inside a character class. White-
space does not include the VT character (code 11). In addition, charac-
ters between an unescaped # outside a character class and the next new-
line, inclusive, are also ignored. This is equivalent to Perl's /x
option, and it can be changed within a pattern by a (?x) option set-
ting.
This option makes it possible to include comments inside complicated
patterns. Note, however, that this applies only to data characters.
Whitespace characters may never appear within special character
sequences in a pattern, for example within the sequence (?( which
introduces a conditional subpattern.
PCRE_EXTRA
This option was invented in order to turn on additional functionality
of PCRE that is incompatible with Perl, but it is currently of very
little use. When set, any backslash in a pattern that is followed by a
letter that has no special meaning causes an error, thus reserving
these combinations for future expansion. By default, as in Perl, a
backslash followed by a letter with no special meaning is treated as a
literal. (Perl can, however, be persuaded to give a warning for this.)
There are at present no other features controlled by this option. It
can also be set by a (?X) option setting within a pattern.
PCRE_FIRSTLINE
If this option is set, an unanchored pattern is required to match
before or at the first newline in the subject string, though the
matched text may continue over the newline.
PCRE_JAVASCRIPT_COMPAT
If this option is set, PCRE's behaviour is changed in some ways so that
it is compatible with JavaScript rather than Perl. The changes are as
follows:
(1) A lone closing square bracket in a pattern causes a compile-time
error, because this is illegal in JavaScript (by default it is treated
as a data character). Thus, the pattern AB]CD becomes illegal when this
option is set.
(2) At run time, a back reference to an unset subpattern group matches
an empty string (by default this causes the current matching alterna-
tive to fail). A pattern such as (\1)(a) succeeds when this option is
set (assuming it can find an "a" in the subject), whereas it fails by
default, for Perl compatibility.
PCRE_MULTILINE
By default, PCRE treats the subject string as consisting of a single
line of characters (even if it actually contains newlines). The "start
of line" metacharacter (^) matches only at the start of the string,
while the "end of line" metacharacter ($) matches only at the end of
the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
is set). This is the same as Perl.
When PCRE_MULTILINE it is set, the "start of line" and "end of line"
constructs match immediately following or immediately before internal
newlines in the subject string, respectively, as well as at the very
start and end. This is equivalent to Perl's /m option, and it can be
changed within a pattern by a (?m) option setting. If there are no new-
lines in a subject string, or no occurrences of ^ or $ in a pattern,
setting PCRE_MULTILINE has no effect.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the default newline definition that was chosen
when PCRE was built. Setting the first or the second specifies that a
newline is indicated by a single character (CR or LF, respectively).
Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
that any of the three preceding sequences should be recognized. Setting
PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
recognized. The Unicode newline sequences are the three just mentioned,
plus the single characters VT (vertical tab, U+000B), FF (formfeed,
U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
(paragraph separator, U+2029). The last two are recognized only in
UTF-8 mode.
The newline setting in the options word uses three bits that are
treated as a number, giving eight possibilities. Currently only six are
used (default plus the five values above). This means that if you set
more than one newline option, the combination may or may not be sensi-
ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
cause an error.
The only time that a line break is specially recognized when compiling
a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
character class is encountered. This indicates a comment that lasts
until after the next line break sequence. In other circumstances, line
break sequences are treated as literal data, except that in
PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
and are therefore ignored.
The newline option that is set at compile time becomes the default that
is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
PCRE_NO_AUTO_CAPTURE
If this option is set, it disables the use of numbered capturing paren-
theses in the pattern. Any opening parenthesis that is not followed by
? behaves as if it were followed by ?: but named parentheses can still
be used for capturing (and they acquire numbers in the usual way).
There is no equivalent of this option in Perl.
PCRE_UNGREEDY
This option inverts the "greediness" of the quantifiers so that they
are not greedy by default, but become greedy if followed by "?". It is
not compatible with Perl. It can also be set by a (?U) option setting
within the pattern.
PCRE_UTF8
This option causes PCRE to regard both the pattern and the subject as
strings of UTF-8 characters instead of single-byte character strings.
However, it is available only when PCRE is built to include UTF-8 sup-
port. If not, the use of this option provokes an error. Details of how
this option changes the behaviour of PCRE are given in the section on
UTF-8 support in the main pcre page.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
automatically checked. There is a discussion about the validity of
UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
bytes is found, pcre_compile() returns an error. If you already know
that your pattern is valid, and you want to skip this check for perfor-
mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
set, the effect of passing an invalid UTF-8 string as a pattern is
undefined. It may cause your program to crash. Note that this option
can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
UTF-8 validity checking of subject strings.
COMPILATION ERROR CODES
The following table lists the error codes than may be returned by
pcre_compile2(), along with the error messages that may be returned by
both compiling functions. As PCRE has developed, some error codes have
fallen out of use. To avoid confusion, they have not been re-used.
0 no error
1 \ at end of pattern
2 \c at end of pattern
3 unrecognized character follows \
4 numbers out of order in {} quantifier
5 number too big in {} quantifier
6 missing terminating ] for character class
7 invalid escape sequence in character class
8 range out of order in character class
9 nothing to repeat
10 [this code is not in use]
11 internal error: unexpected repeat
12 unrecognized character after (? or (?-
13 POSIX named classes are supported only within a class
14 missing )
15 reference to non-existent subpattern
16 erroffset passed as NULL
17 unknown option bit(s) set
18 missing ) after comment
19 [this code is not in use]
20 regular expression is too large
21 failed to get memory
22 unmatched parentheses
23 internal error: code overflow
24 unrecognized character after (?<
25 lookbehind assertion is not fixed length
26 malformed number or name after (?(
27 conditional group contains more than two branches
28 assertion expected after (?(
29 (?R or (?[+-]digits must be followed by )
30 unknown POSIX class name
31 POSIX collating elements are not supported
32 this version of PCRE is not compiled with PCRE_UTF8 support
33 [this code is not in use]
34 character value in \x{...} sequence is too large
35 invalid condition (?(0)
36 \C not allowed in lookbehind assertion
37 PCRE does not support \L, \l, \N, \U, or \u
38 number after (?C is > 255
39 closing ) for (?C expected
40 recursive call could loop indefinitely
41 unrecognized character after (?P
42 syntax error in subpattern name (missing terminator)
43 two named subpatterns have the same name
44 invalid UTF-8 string
45 support for \P, \p, and \X has not been compiled
46 malformed \P or \p sequence
47 unknown property name after \P or \p
48 subpattern name is too long (maximum 32 characters)
49 too many named subpatterns (maximum 10000)
50 [this code is not in use]
51 octal value is greater than \377 (not in UTF-8 mode)
52 internal error: overran compiling workspace
53 internal error: previously-checked referenced subpattern not
found
54 DEFINE group contains more than one branch
55 repeating a DEFINE group is not allowed
56 inconsistent NEWLINE options
57 \g is not followed by a braced, angle-bracketed, or quoted
name/number or by a plain number
58 a numbered reference must not be zero
59 (*VERB) with an argument is not supported
60 (*VERB) not recognized
61 number is too big
62 subpattern name expected
63 digit expected after (?+
64 ] is an invalid data character in JavaScript compatibility mode
The numbers 32 and 10000 in errors 48 and 49 are defaults; different
values may be used if the limits were changed when PCRE was built.
STUDYING A PATTERN
pcre_extra *pcre_study(const pcre *code, int options
const char **errptr);
If a compiled pattern is going to be used several times, it is worth
spending more time analyzing it in order to speed up the time taken for
matching. The function pcre_study() takes a pointer to a compiled pat-
tern as its first argument. If studying the pattern produces additional
information that will help speed up matching, pcre_study() returns a
pointer to a pcre_extra block, in which the study_data field points to
the results of the study.
The returned value from pcre_study() can be passed directly to
pcre_exec(). However, a pcre_extra block also contains other fields
that can be set by the caller before the block is passed; these are
described below in the section on matching a pattern.
If studying the pattern does not produce any additional information
pcre_study() returns NULL. In that circumstance, if the calling program
wants to pass any of the other fields to pcre_exec(), it must set up
its own pcre_extra block.
The second argument of pcre_study() contains option bits. At present,
no options are defined, and this argument should always be zero.
The third argument for pcre_study() is a pointer for an error message.
If studying succeeds (even if no data is returned), the variable it
points to is set to NULL. Otherwise it is set to point to a textual
error message. This is a static string that is part of the library. You
must not try to free it. You should test the error pointer for NULL
after calling pcre_study(), to be sure that it has run successfully.
This is a typical call to pcre_study():
pcre_extra *pe;
pe = pcre_study(
re, /* result of pcre_compile() */
0, /* no options exist */
&error;); /* set to NULL or points to a message */
At present, studying a pattern is useful only for non-anchored patterns
that do not have a single fixed starting character. A bitmap of possi-
ble starting bytes is created.
LOCALE SUPPORT
PCRE handles caseless matching, and determines whether characters are
letters, digits, or whatever, by reference to a set of tables, indexed
by character value. When running in UTF-8 mode, this applies only to
characters with codes less than 128. Higher-valued codes never match
escapes such as \w or \d, but can be tested with \p if PCRE is built
with Unicode character property support. The use of locales with Uni-
code is discouraged. If you are handling characters with codes greater
than 128, you should either use UTF-8 and Unicode, or use locales, but
not try to mix the two.
PCRE contains an internal set of tables that are used when the final
argument of pcre_compile() is NULL. These are sufficient for many
applications. Normally, the internal tables recognize only ASCII char-
acters. However, when PCRE is built, it is possible to cause the inter-
nal tables to be rebuilt in the default "C" locale of the local system,
which may cause them to be different.
The internal tables can always be overridden by tables supplied by the
application that calls PCRE. These may be created in a different locale
from the default. As more and more applications change to using Uni-
code, the need for this locale support is expected to die away.
External tables are built by calling the pcre_maketables() function,
which has no arguments, in the relevant locale. The result can then be
passed to pcre_compile() or pcre_exec() as often as necessary. For
example, to build and use tables that are appropriate for the French
locale (where accented characters with values greater than 128 are
treated as letters), the following code could be used:
setlocale(LC_CTYPE, "fr_FR");
tables = pcre_maketables();
re = pcre_compile(..., tables);
The locale name "fr_FR" is used on Linux and other Unix-like systems;
if you are using Windows, the name for the French locale is "french".
When pcre_maketables() runs, the tables are built in memory that is
obtained via pcre_malloc. It is the caller's responsibility to ensure
that the memory containing the tables remains available for as long as
it is needed.
The pointer that is passed to pcre_compile() is saved with the compiled
pattern, and the same tables are used via this pointer by pcre_study()
and normally also by pcre_exec(). Thus, by default, for any single pat-
tern, compilation, studying and matching all happen in the same locale,
but different patterns can be compiled in different locales.
It is possible to pass a table pointer or NULL (indicating the use of
the internal tables) to pcre_exec(). Although not intended for this
purpose, this facility could be used to match a pattern in a different
locale from the one in which it was compiled. Passing table pointers at
run time is discussed below in the section on matching a pattern.
INFORMATION ABOUT A PATTERN
int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);
The pcre_fullinfo() function returns information about a compiled pat-
tern. It replaces the obsolete pcre_info() function, which is neverthe-
less retained for backwards compability (and is documented below).
The first argument for pcre_fullinfo() is a pointer to the compiled
pattern. The second argument is the result of pcre_study(), or NULL if
the pattern was not studied. The third argument specifies which piece
of information is required, and the fourth argument is a pointer to a
variable to receive the data. The yield of the function is zero for
success, or one of the following negative numbers:
PCRE_ERROR_NULL the argument code was NULL
the argument where was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
PCRE_ERROR_BADOPTION the value of what was invalid
The "magic number" is placed at the start of each compiled pattern as
an simple check against passing an arbitrary memory pointer. Here is a
typical call of pcre_fullinfo(), to obtain the length of the compiled
pattern:
int rc;
size_t length;
rc = pcre_fullinfo(
re, /* result of pcre_compile() */
pe, /* result of pcre_study(), or NULL */
PCRE_INFO_SIZE, /* what is required */
&length;); /* where to put the data */
The possible values for the third argument are defined in pcre.h, and
are as follows:
PCRE_INFO_BACKREFMAX
Return the number of the highest back reference in the pattern. The
fourth argument should point to an int variable. Zero is returned if
there are no back references.
PCRE_INFO_CAPTURECOUNT
Return the number of capturing subpatterns in the pattern. The fourth
argument should point to an int variable.
PCRE_INFO_DEFAULT_TABLES
Return a pointer to the internal default character tables within PCRE.
The fourth argument should point to an unsigned char * variable. This
information call is provided for internal use by the pcre_study() func-
tion. External callers can cause PCRE to use its internal tables by
passing a NULL table pointer.
PCRE_INFO_FIRSTBYTE
Return information about the first byte of any matched string, for a
non-anchored pattern. The fourth argument should point to an int vari-
able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
is still recognized for backwards compatibility.)
If there is a fixed first byte, for example, from a pattern such as
(cat|cow|coyote), its value is returned. Otherwise, if either
(a) the pattern was compiled with the PCRE_MULTILINE option, and every
branch starts with "^", or
(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
set (if it were set, the pattern would be anchored),
-1 is returned, indicating that the pattern matches only at the start
of a subject string or after any newline within the string. Otherwise
-2 is returned. For anchored patterns, -2 is returned.
PCRE_INFO_FIRSTTABLE
If the pattern was studied, and this resulted in the construction of a
256-bit table indicating a fixed set of bytes for the first byte in any
matching string, a pointer to the table is returned. Otherwise NULL is
returned. The fourth argument should point to an unsigned char * vari-
able.
PCRE_INFO_HASCRORLF
Return 1 if the pattern contains any explicit matches for CR or LF
characters, otherwise 0. The fourth argument should point to an int
variable. An explicit match is either a literal CR or LF character, or
\r or \n.
PCRE_INFO_JCHANGED
Return 1 if the (?J) or (?-J) option setting is used in the pattern,
otherwise 0. The fourth argument should point to an int variable. (?J)
and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
PCRE_INFO_LASTLITERAL
Return the value of the rightmost literal byte that must exist in any
matched string, other than at its start, if such a byte has been
recorded. The fourth argument should point to an int variable. If there
is no such byte, -1 is returned. For anchored patterns, a last literal
byte is recorded only if it follows something of variable length. For
example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
/^a\dz\d/ the returned value is -1.
PCRE_INFO_NAMECOUNT
PCRE_INFO_NAMEENTRYSIZE
PCRE_INFO_NAMETABLE
PCRE supports the use of named as well as numbered capturing parenthe-
ses. The names are just an additional way of identifying the parenthe-
ses, which still acquire numbers. Several convenience functions such as
pcre_get_named_substring() are provided for extracting captured sub-
strings by name. It is also possible to extract the data directly, by
first converting the name to a number in order to access the correct
pointers in the output vector (described with pcre_exec() below). To do
the conversion, you need to use the name-to-number map, which is
described by these three values.
The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
of each entry; both of these return an int value. The entry size
depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
a pointer to the first entry of the table (a pointer to char). The
first two bytes of each entry are the number of the capturing parenthe-
sis, most significant byte first. The rest of the entry is the corre-
sponding name, zero terminated. The names are in alphabetical order.
When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
theses numbers. For example, consider the following pattern (assume
PCRE_EXTENDED is set, so white space - including newlines - is
ignored):
(?<date> (?<year>(\d\d)?\d\d) -
(?<month>\d\d) - (?<day>\d\d) )
There are four named subpatterns, so the table has four entries, and
each entry in the table is eight bytes long. The table is as follows,
with non-printing bytes shows in hexadecimal, and undefined bytes shown
as ??:
00 01 d a t e 00 ??
00 05 d a y 00 ?? ??
00 04 m o n t h 00
00 02 y e a r 00 ??
When writing code to extract data from named subpatterns using the
name-to-number map, remember that the length of the entries is likely
to be different for each compiled pattern.
PCRE_INFO_OKPARTIAL
Return 1 if the pattern can be used for partial matching, otherwise 0.
The fourth argument should point to an int variable. The pcrepartial
documentation lists the restrictions that apply to patterns when par-
tial matching is used.
PCRE_INFO_OPTIONS
Return a copy of the options with which the pattern was compiled. The
fourth argument should point to an unsigned long int variable. These
option bits are those specified in the call to pcre_compile(), modified
by any top-level option settings at the start of the pattern itself. In
other words, they are the options that will be in force when matching
starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
and PCRE_EXTENDED.
A pattern is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:
^ unless PCRE_MULTILINE is set
\A always
\G always
.* if PCRE_DOTALL is set and there are no back
references to the subpattern in which .* appears
For such patterns, the PCRE_ANCHORED bit is set in the options returned
by pcre_fullinfo().
PCRE_INFO_SIZE
Return the size of the compiled pattern, that is, the value that was
passed as the argument to pcre_malloc() when PCRE was getting memory in
which to place the compiled data. The fourth argument should point to a
size_t variable.
PCRE_INFO_STUDYSIZE
Return the size of the data block pointed to by the study_data field in
a pcre_extra block. That is, it is the value that was passed to
pcre_malloc() when PCRE was getting memory into which to place the data
created by pcre_study(). The fourth argument should point to a size_t
variable.
OBSOLETE INFO FUNCTION
int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
The pcre_info() function is now obsolete because its interface is too
restrictive to return all the available data about a compiled pattern.
New programs should use pcre_fullinfo() instead. The yield of
pcre_info() is the number of capturing subpatterns, or one of the fol-
lowing negative numbers:
PCRE_ERROR_NULL the argument code was NULL
PCRE_ERROR_BADMAGIC the "magic number" was not found
If the optptr argument is not NULL, a copy of the options with which
the pattern was compiled is placed in the integer it points to (see
PCRE_INFO_OPTIONS above).
If the pattern is not anchored and the firstcharptr argument is not
NULL, it is used to pass back information about the first character of
any matched string (see PCRE_INFO_FIRSTBYTE above).
REFERENCE COUNTS
int pcre_refcount(pcre *code, int adjust);
The pcre_refcount() function is used to maintain a reference count in
the data block that contains a compiled pattern. It is provided for the
benefit of applications that operate in an object-oriented manner,
where different parts of the application may be using the same compiled
pattern, but you want to free the block when they are all done.
When a pattern is compiled, the reference count field is initialized to
zero. It is changed only by calling this function, whose action is to
add the adjust value (which may be positive or negative) to it. The
yield of the function is the new value. However, the value of the count
is constrained to lie between 0 and 65535, inclusive. If the new value
is outside these limits, it is forced to the appropriate limit value.
Except when it is zero, the reference count is not correctly preserved
if a pattern is compiled on one host and then transferred to a host
whose byte-order is different. (This seems a highly unlikely scenario.)
MATCHING A PATTERN: THE TRADITIONAL FUNCTION
int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);
The function pcre_exec() is called to match a subject string against a
compiled pattern, which is passed in the code argument. If the pattern
has been studied, the result of the study should be passed in the extra
argument. This function is the main matching facility of the library,
and it operates in a Perl-like manner. For specialist use there is also
an alternative matching function, which is described below in the sec-
tion about the pcre_dfa_exec() function.
In most applications, the pattern will have been compiled (and option-
ally studied) in the same process that calls pcre_exec(). However, it
is possible to save compiled patterns and study data, and then use them
later in different processes, possibly even on different hosts. For a
discussion about this, see the pcreprecompile documentation.
Here is an example of a simple call to pcre_exec():
int rc;
int ovector[30];
rc = pcre_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
30); /* number of elements (NOT size in bytes) */
Extra data for pcre_exec()
If the extra argument is not NULL, it must point to a pcre_extra data
block. The pcre_study() function returns such a block (when it doesn't
return NULL), but you can also create one for yourself, and pass addi-
tional information in it. The pcre_extra block contains the following
fields (not necessarily in this order):
unsigned long int flags;
void *study_data;
unsigned long int match_limit;
unsigned long int match_limit_recursion;
void *callout_data;
const unsigned char *tables;
The flags field is a bitmap that specifies which of the other fields
are set. The flag bits are:
PCRE_EXTRA_STUDY_DATA
PCRE_EXTRA_MATCH_LIMIT
PCRE_EXTRA_MATCH_LIMIT_RECURSION
PCRE_EXTRA_CALLOUT_DATA
PCRE_EXTRA_TABLES
Other flag bits should be set to zero. The study_data field is set in
the pcre_extra block that is returned by pcre_study(), together with
the appropriate flag bit. You should not set this yourself, but you may
add to the block by setting the other fields and their corresponding
flag bits.
The match_limit field provides a means of preventing PCRE from using up
a vast amount of resources when running patterns that are not going to
match, but which have a very large number of possibilities in their
search trees. The classic example is the use of nested unlimited
repeats.
Internally, PCRE uses a function called match() which it calls repeat-
edly (sometimes recursively). The limit set by match_limit is imposed
on the number of times this function is called during a match, which
has the effect of limiting the amount of backtracking that can take
place. For patterns that are not anchored, the count restarts from zero
for each position in the subject string.
The default value for the limit can be set when PCRE is built; the
default default is 10 million, which handles all but the most extreme
cases. You can override the default by suppling pcre_exec() with a
pcre_extra block in which match_limit is set, and
PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
The match_limit_recursion field is similar to match_limit, but instead
of limiting the total number of times that match() is called, it limits
the depth of recursion. The recursion depth is a smaller number than
the total number of calls, because not all calls to match() are recur-
sive. This limit is of use only if it is set smaller than match_limit.
Limiting the recursion depth limits the amount of stack that can be
used, or, when PCRE has been compiled to use memory on the heap instead
of the stack, the amount of heap memory that can be used.
The default value for match_limit_recursion can be set when PCRE is
built; the default default is the same value as the default for
match_limit. You can override the default by suppling pcre_exec() with
a pcre_extra block in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
The pcre_callout field is used in conjunction with the "callout" fea-
ture, which is described in the pcrecallout documentation.
The tables field is used to pass a character tables pointer to
pcre_exec(); this overrides the value that is stored with the compiled
pattern. A non-NULL value is stored with the compiled pattern only if
custom tables were supplied to pcre_compile() via its tableptr argu-
ment. If NULL is passed to pcre_exec() using this mechanism, it forces
PCRE's internal tables to be used. This facility is helpful when re-
using patterns that have been saved after compiling with an external
set of tables, because the external tables might be at a different
address when pcre_exec() is called. See the pcreprecompile documenta-
tion for a discussion of saving compiled patterns for later use.
Option bits for pcre_exec()
The unused bits of the options argument for pcre_exec() must be zero.
The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
PCRE_PARTIAL.
PCRE_ANCHORED
The PCRE_ANCHORED option limits pcre_exec() to matching at the first
matching position. If a pattern was compiled with PCRE_ANCHORED, or
turned out to be anchored by virtue of its contents, it cannot be made
unachored at matching time.
PCRE_BSR_ANYCRLF
PCRE_BSR_UNICODE
These options (which are mutually exclusive) control what the \R escape
sequence matches. The choice is either to match only CR, LF, or CRLF,
or to match any Unicode newline sequence. These options override the
choice that was made or defaulted when the pattern was compiled.
PCRE_NEWLINE_CR
PCRE_NEWLINE_LF
PCRE_NEWLINE_CRLF
PCRE_NEWLINE_ANYCRLF
PCRE_NEWLINE_ANY
These options override the newline definition that was chosen or
defaulted when the pattern was compiled. For details, see the descrip-
tion of pcre_compile() above. During matching, the newline choice
affects the behaviour of the dot, circumflex, and dollar metacharac-
ters. It may also alter the way the match position is advanced after a
match failure for an unanchored pattern.
When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
set, and a match attempt for an unanchored pattern fails when the cur-
rent position is at a CRLF sequence, and the pattern contains no
explicit matches for CR or LF characters, the match position is
advanced by two characters instead of one, in other words, to after the
CRLF.
The above rule is a compromise that makes the most common cases work as
expected. For example, if the pattern is .+A (and the PCRE_DOTALL
option is not set), it does not match the string "\r\nA" because, after
failing at the start, it skips both the CR and the LF before retrying.
However, the pattern [\r\n]A does match that string, because it con-
tains an explicit CR or LF reference, and so advances only by one char-
acter after the first failure.
An explicit match for CR of LF is either a literal appearance of one of
those characters, or one of the \r or \n escape sequences. Implicit
matches such as [^X] do not count, nor does \s (which includes CR and
LF in the characters that it matches).
Notwithstanding the above, anomalous effects may still occur when CRLF
is a valid newline sequence and explicit \r or \n escapes appear in the
pattern.
PCRE_NOTBOL
This option specifies that first character of the subject string is not
the beginning of a line, so the circumflex metacharacter should not
match before it. Setting this without PCRE_MULTILINE (at compile time)
causes circumflex never to match. This option affects only the behav-
iour of the circumflex metacharacter. It does not affect \A.
PCRE_NOTEOL
This option specifies that the end of the subject string is not the end
of a line, so the dollar metacharacter should not match it nor (except
in multiline mode) a newline immediately before it. Setting this with-
out PCRE_MULTILINE (at compile time) causes dollar never to match. This
option affects only the behaviour of the dollar metacharacter. It does
not affect \Z or \z.
PCRE_NOTEMPTY
An empty string is not considered to be a valid match if this option is
set. If there are alternatives in the pattern, they are tried. If all
the alternatives match the empty string, the entire match fails. For
example, if the pattern
a?b?
is applied to a string not beginning with "a" or "b", it matches the
empty string at the start of the subject. With PCRE_NOTEMPTY set, this
match is not valid, so PCRE searches further into the string for occur-
rences of "a" or "b".
Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
cial case of a pattern match of the empty string within its split()
function, and when using the /g modifier. It is possible to emulate
Perl's behaviour after matching a null string by first trying the match
again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
if that fails by advancing the starting offset (see below) and trying
an ordinary match again. There is some code that demonstrates how to do
this in the pcredemo.c sample program.
PCRE_NO_UTF8_CHECK
When PCRE_UTF8 is set at compile time, the validity of the subject as a
UTF-8 string is automatically checked when pcre_exec() is subsequently
called. The value of startoffset is also checked to ensure that it
points to the start of a UTF-8 character. There is a discussion about
the validity of UTF-8 strings in the section on UTF-8 support in the
main pcre page. If an invalid UTF-8 sequence of bytes is found,
pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
If you already know that your subject is valid, and you want to skip
these checks for performance reasons, you can set the
PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
do this for the second and subsequent calls to pcre_exec() if you are
making repeated calls to find all the matches in a single subject
string. However, you should be sure that the value of startoffset
points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
set, the effect of passing an invalid UTF-8 string as a subject, or a
value of startoffset that does not point to the start of a UTF-8 char-
acter, is undefined. Your program may crash.
PCRE_PARTIAL
This option turns on the partial matching feature. If the subject
string fails to match the pattern, but at some point during the match-
ing process the end of the subject was reached (that is, the subject
partially matches the pattern and the failure to match occurred only
because there were not enough subject characters), pcre_exec() returns
PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
used, there are restrictions on what may appear in the pattern. These
are discussed in the pcrepartial documentation.
The string to be matched by pcre_exec()
The subject string is passed to pcre_exec() as a pointer in subject, a
length (in bytes) in length, and a starting byte offset in startoffset.
In UTF-8 mode, the byte offset must point to the start of a UTF-8 char-
acter. Unlike the pattern string, the subject may contain binary zero
bytes. When the starting offset is zero, the search for a match starts
at the beginning of the subject, and this is by far the most common
case.
A non-zero starting offset is useful when searching for another match
in the same subject by calling pcre_exec() again after a previous suc-
cess. Setting startoffset differs from just passing over a shortened
string and setting PCRE_NOTBOL in the case of a pattern that begins
with any kind of lookbehind. For example, consider the pattern
\Biss\B
which finds occurrences of "iss" in the middle of words. (\B matches
only if the current position in the subject is not a word boundary.)
When applied to the string "Mississipi" the first call to pcre_exec()
finds the first occurrence. If pcre_exec() is called again with just
the remainder of the subject, namely "issipi", it does not match,
because \B is always false at the start of the subject, which is deemed
to be a word boundary. However, if pcre_exec() is passed the entire
string again, but with startoffset set to 4, it finds the second occur-
rence of "iss" because it is able to look behind the starting point to
discover that it is preceded by a letter.
If a non-zero starting offset is passed when the pattern is anchored,
one attempt to match at the given offset is made. This can only succeed
if the pattern does not require the match to be at the start of the
subject.
How pcre_exec() returns captured substrings
In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by
parts of the pattern. Following the usage in Jeffrey Friedl's book,
this is called "capturing" in what follows, and the phrase "capturing
subpattern" is used for a fragment of a pattern that picks out a sub-
string. PCRE supports several other kinds of parenthesized subpattern
that do not cause substrings to be captured.
Captured substrings are returned to the caller via a vector of integers
whose address is passed in ovector. The number of elements in the vec-
tor is passed in ovecsize, which must be a non-negative number. Note:
this argument is NOT the size of ovector in bytes.
The first two-thirds of the vector is used to pass back captured sub-
strings, each substring using a pair of integers. The remaining third
of the vector is used as workspace by pcre_exec() while matching cap-
turing subpatterns, and is not available for passing back information.
The number passed in ovecsize should always be a multiple of three. If
it is not, it is rounded down.
When a match is successful, information about captured substrings is
returned in pairs of integers, starting at the beginning of ovector,
and continuing up to two-thirds of its length at the most. The first
element of each pair is set to the byte offset of the first character
in a substring, and the second is set to the byte offset of the first
character after the end of a substring. Note: these values are always
byte offsets, even in UTF-8 mode. They are not character counts.
The first pair of integers, ovector[0] and ovector[1], identify the
portion of the subject string matched by the entire pattern. The next
pair is used for the first capturing subpattern, and so on. The value
returned by pcre_exec() is one more than the highest numbered pair that
has been set. For example, if two substrings have been captured, the
returned value is 3. If there are no capturing subpatterns, the return
value from a successful match is 1, indicating that just the first pair
of offsets has been set.
If a capturing subpattern is matched repeatedly, it is the last portion
of the string that it matched that is returned.
If the vector is too small to hold all the captured substring offsets,
it is used as far as possible (up to two-thirds of its length), and the
function returns a value of zero. If the substring offsets are not of
interest, pcre_exec() may be called with ovector passed as NULL and
ovecsize as zero. However, if the pattern contains back references and
the ovector is not big enough to remember the related substrings, PCRE
has to get additional memory for use during matching. Thus it is usu-
ally advisable to supply an ovector.
The pcre_info() function can be used to find out how many capturing
subpatterns there are in a compiled pattern. The smallest size for
ovector that will allow for n captured substrings, in addition to the
offsets of the substring matched by the whole pattern, is (n+1)*3.
It is possible for capturing subpattern number n+1 to match some part
of the subject when subpattern n has not been used at all. For example,
if the string "abc" is matched against the pattern (a|(z))(bc) the
return from the function is 4, and subpatterns 1 and 3 are matched, but
2 is not. When this happens, both values in the offset pairs corre-
sponding to unused subpatterns are set to -1.
Offset values that correspond to unused subpatterns at the end of the
expression are also set to -1. For example, if the string "abc" is
matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
matched. The return from the function is 2, because the highest used
capturing subpattern number is 1. However, you can refer to the offsets
for the second and third capturing subpatterns if you wish (assuming
the vector is large enough, of course).
Some convenience functions are provided for extracting the captured
substrings as separate strings. These are described below.
Error return values from pcre_exec()
If pcre_exec() fails, it returns a negative number. The following are
defined in the header file:
PCRE_ERROR_NOMATCH (-1)
The subject string did not match the pattern.
PCRE_ERROR_NULL (-2)
Either code or subject was passed as NULL, or ovector was NULL and
ovecsize was not zero.
PCRE_ERROR_BADOPTION (-3)
An unrecognized bit was set in the options argument.
PCRE_ERROR_BADMAGIC (-4)
PCRE stores a 4-byte "magic number" at the start of the compiled code,
to catch the case when it is passed a junk pointer and to detect when a
pattern that was compiled in an environment of one endianness is run in
an environment with the other endianness. This is the error that PCRE
gives when the magic number is not present.
PCRE_ERROR_UNKNOWN_OPCODE (-5)
While running the pattern match, an unknown item was encountered in the
compiled pattern. This error could be caused by a bug in PCRE or by
overwriting of the compiled pattern.
PCRE_ERROR_NOMEMORY (-6)
If a pattern contains back references, but the ovector that is passed
to pcre_exec() is not big enough to remember the referenced substrings,
PCRE gets a block of memory at the start of matching to use for this
purpose. If the call via pcre_malloc() fails, this error is given. The
memory is automatically freed at the end of matching.
PCRE_ERROR_NOSUBSTRING (-7)
This error is used by the pcre_copy_substring(), pcre_get_substring(),
and pcre_get_substring_list() functions (see below). It is never
returned by pcre_exec().
PCRE_ERROR_MATCHLIMIT (-8)
The backtracking limit, as specified by the match_limit field in a
pcre_extra structure (or defaulted) was reached. See the description
above.
PCRE_ERROR_CALLOUT (-9)
This error is never generated by pcre_exec() itself. It is provided for
use by callout functions that want to yield a distinctive error code.
See the pcrecallout documentation for details.
PCRE_ERROR_BADUTF8 (-10)
A string that contains an invalid UTF-8 byte sequence was passed as a
subject.
PCRE_ERROR_BADUTF8_OFFSET (-11)
The UTF-8 byte sequence that was passed as a subject was valid, but the
value of startoffset did not point to the beginning of a UTF-8 charac-
ter.
PCRE_ERROR_PARTIAL (-12)
The subject string did not match, but it did match partially. See the
pcrepartial documentation for details of partial matching.
PCRE_ERROR_BADPARTIAL (-13)
The PCRE_PARTIAL option was used with a compiled pattern containing
items that are not supported for partial matching. See the pcrepartial
documentation for details of partial matching.
PCRE_ERROR_INTERNAL (-14)
An unexpected internal error has occurred. This error could be caused
by a bug in PCRE or by overwriting of the compiled pattern.
PCRE_ERROR_BADCOUNT (-15)
This error is given if the value of the ovecsize argument is negative.
PCRE_ERROR_RECURSIONLIMIT (-21)
The internal recursion limit, as specified by the match_limit_recursion
field in a pcre_extra structure (or defaulted) was reached. See the
description above.
PCRE_ERROR_BADNEWLINE (-23)
An invalid combination of PCRE_NEWLINE_xxx options was given.
Error numbers -16 to -20 and -22 are not used by pcre_exec().
EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);
int pcre_get_substring(const char *subject, int *ovector,
int stringcount, int stringnumber,
const char **stringptr);
int pcre_get_substring_list(const char *subject,
int *ovector, int stringcount, const char ***listptr);
Captured substrings can be accessed directly by using the offsets
returned by pcre_exec() in ovector. For convenience, the functions
pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
string_list() are provided for extracting captured substrings as new,
separate, zero-terminated strings. These functions identify substrings
by number. The next section describes functions for extracting named
substrings.
A substring that contains a binary zero is correctly extracted and has
a further zero added on the end, but the result is not, of course, a C
string. However, you can process such a string by referring to the
length that is returned by pcre_copy_substring() and pcre_get_sub-
string(). Unfortunately, the interface to pcre_get_substring_list() is
not adequate for handling strings containing binary zeros, because the
end of the final string is not independently indicated.
The first three arguments are the same for all three of these func-
tions: subject is the subject string that has just been successfully
matched, ovector is a pointer to the vector of integer offsets that was
passed to pcre_exec(), and stringcount is the number of substrings that
were captured by the match, including the substring that matched the
entire regular expression. This is the value returned by pcre_exec() if
it is greater than zero. If pcre_exec() returned zero, indicating that
it ran out of space in ovector, the value passed as stringcount should
be the number of elements in the vector divided by three.
The functions pcre_copy_substring() and pcre_get_substring() extract a
single substring, whose number is given as stringnumber. A value of
zero extracts the substring that matched the entire pattern, whereas
higher values extract the captured substrings. For pcre_copy_sub-
string(), the string is placed in buffer, whose length is given by
buffersize, while for pcre_get_substring() a new block of memory is
obtained via pcre_malloc, and its address is returned via stringptr.
The yield of the function is the length of the string, not including
the terminating zero, or one of these error codes:
PCRE_ERROR_NOMEMORY (-6)
The buffer was too small for pcre_copy_substring(), or the attempt to
get memory failed for pcre_get_substring().
PCRE_ERROR_NOSUBSTRING (-7)
There is no substring whose number is stringnumber.
The pcre_get_substring_list() function extracts all available sub-
strings and builds a list of pointers to them. All this is done in a
single block of memory that is obtained via pcre_malloc. The address of
the memory block is returned via listptr, which is also the start of
the list of string pointers. The end of the list is marked by a NULL
pointer. The yield of the function is zero if all went well, or the
error code
PCRE_ERROR_NOMEMORY (-6)
if the attempt to get the memory block failed.
When any of these functions encounter a substring that is unset, which
can happen when capturing subpattern number n+1 matches some part of
the subject, but subpattern n has not been used at all, they return an
empty string. This can be distinguished from a genuine zero-length sub-
string by inspecting the appropriate offset in ovector, which is nega-
tive for unset substrings.
The two convenience functions pcre_free_substring() and pcre_free_sub-
string_list() can be used to free the memory returned by a previous
call of pcre_get_substring() or pcre_get_substring_list(), respec-
tively. They do nothing more than call the function pointed to by
pcre_free, which of course could be called directly from a C program.
However, PCRE is used in some situations where it is linked via a spe-
cial interface to another programming language that cannot use
pcre_free directly; it is for these cases that the functions are pro-
vided.
EXTRACTING CAPTURED SUBSTRINGS BY NAME
int pcre_get_stringnumber(const pcre *code,
const char *name);
int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);
int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);
To extract a substring by name, you first have to find associated num-
ber. For example, for this pattern
(a+)b(?<xxx>\d+)...
the number of the subpattern called "xxx" is 2. If the name is known to
be unique (PCRE_DUPNAMES was not set), you can find the number from the
name by calling pcre_get_stringnumber(). The first argument is the com-
piled pattern, and the second is the name. The yield of the function is
the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
subpattern of that name.
Given the number, you can extract the substring directly, or use one of
the functions described in the previous section. For convenience, there
are also two functions that do the whole job.
Most of the arguments of pcre_copy_named_substring() and
pcre_get_named_substring() are the same as those for the similarly
named functions that extract by number. As these are described in the
previous section, they are not re-described here. There are just two
differences:
First, instead of a substring number, a substring name is given. Sec-
ond, there is an extra argument, given at the start, which is a pointer
to the compiled pattern. This is needed in order to gain access to the
name-to-number translation table.
These functions call pcre_get_stringnumber(), and if it succeeds, they
then call pcre_copy_substring() or pcre_get_substring(), as appropri-
ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
behaviour may not be what you want (see the next section).
DUPLICATE SUBPATTERN NAMES
int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);
When a pattern is compiled with the PCRE_DUPNAMES option, names for
subpatterns are not required to be unique. Normally, patterns with
duplicate names are such that in any one match, only one of the named
subpatterns participates. An example is shown in the pcrepattern docu-
mentation.
When duplicates are present, pcre_copy_named_substring() and
pcre_get_named_substring() return the first substring corresponding to
the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
(-7) is returned; no data is returned. The pcre_get_stringnumber()
function returns one of the numbers that are associated with the name,
but it is not defined which it is.
If you want to get full details of all captured substrings for a given
name, you must use the pcre_get_stringtable_entries() function. The
first argument is the compiled pattern, and the second is the name. The
third and fourth are pointers to variables which are updated by the
function. After it has run, they point to the first and last entries in
the name-to-number table for the given name. The function itself
returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
there are none. The format of the table is described above in the sec-
tion entitled Information about a pattern. Given all the relevant
entries for the name, you can extract each of their numbers, and hence
the captured data, if any.
FINDING ALL POSSIBLE MATCHES
The traditional matching function uses a similar algorithm to Perl,
which stops when it finds the first match, starting at a given point in
the subject. If you want to find all possible matches, or the longest
possible match, consider using the alternative matching function (see
below) instead. If you cannot use the alternative function, but still
need to find all possible matches, you can kludge it up by making use
of the callout facility, which is described in the pcrecallout documen-
tation.
What you have to do is to insert a callout right at the end of the pat-
tern. When your callout function is called, extract and save the cur-
rent matched substring. Then return 1, which forces pcre_exec() to
backtrack and try other alternatives. Ultimately, when it runs out of
matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);
The function pcre_dfa_exec() is called to match a subject string
against a compiled pattern, using a matching algorithm that scans the
subject string just once, and does not backtrack. This has different
characteristics to the normal algorithm, and is not compatible with
Perl. Some of the features of PCRE patterns are not supported. Never-
theless, there are times when this kind of matching can be useful. For
a discussion of the two matching algorithms, see the pcrematching docu-
mentation.
The arguments for the pcre_dfa_exec() function are the same as for
pcre_exec(), plus two extras. The ovector argument is used in a differ-
ent way, and this is described below. The other common arguments are
used in the same way as for pcre_exec(), so their description is not
repeated here.
The two additional arguments provide workspace for the function. The
workspace vector should contain at least 20 elements. It is used for
keeping track of multiple paths through the pattern tree. More
workspace will be needed for patterns and subjects where there are a
lot of potential matches.
Here is an example of a simple call to pcre_dfa_exec():
int rc;
int ovector[10];
int wspace[20];
rc = pcre_dfa_exec(
re, /* result of pcre_compile() */
NULL, /* we didn't study the pattern */
"some string", /* the subject string */
11, /* the length of the subject string */
0, /* start at offset 0 in the subject */
0, /* default options */
ovector, /* vector of integers for substring information */
10, /* number of elements (NOT size in bytes) */
wspace, /* working space vector */
20); /* number of elements (NOT size in bytes) */
Option bits for pcre_dfa_exec()
The unused bits of the options argument for pcre_dfa_exec() must be
zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
three of these are the same as for pcre_exec(), so their description is
not repeated here.
PCRE_PARTIAL
This has the same general effect as it does for pcre_exec(), but the
details are slightly different. When PCRE_PARTIAL is set for
pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
been no complete matches, but there is still at least one matching pos-
sibility. The portion of the string that provided the partial match is
set as the first matching string.
PCRE_DFA_SHORTEST
Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
stop as soon as it has found one match. Because of the way the alterna-
tive algorithm works, this is necessarily the shortest possible match
at the first possible matching point in the subject string.
PCRE_DFA_RESTART
When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
returns a partial match, it is possible to call it again, with addi-
tional subject characters, and have it continue with the same match.
The PCRE_DFA_RESTART option requests this action; when it is set, the
workspace and wscount options must reference the same vector as before
because data about the match so far is left in them after a partial
match. There is more discussion of this facility in the pcrepartial
documentation.
Successful returns from pcre_dfa_exec()
When pcre_dfa_exec() succeeds, it may have matched more than one sub-
string in the subject. Note, however, that all the matches from one run
of the function start at the same point in the subject. The shorter
matches are all initial substrings of the longer matches. For example,
if the pattern
<.*>
is matched against the string
This is <something> <something else> <something further> no more
the three matched strings are
<something>
<something> <something else>
<something> <something else> <something further>
On success, the yield of the function is a number greater than zero,
which is the number of matched substrings. The substrings themselves
are returned in ovector. Each string uses two elements; the first is
the offset to the start, and the second is the offset to the end. In
fact, all the strings have the same start offset. (Space could have
been saved by giving this only once, but it was decided to retain some
compatibility with the way pcre_exec() returns data, even though the
meaning of the strings is different.)
The strings are returned in reverse order of length; that is, the long-
est matching string is given first. If there were too many matches to
fit into ovector, the yield of the function is zero, and the vector is
filled with the longest matches.
Error returns from pcre_dfa_exec()
The pcre_dfa_exec() function returns a negative number when it fails.
Many of the errors are the same as for pcre_exec(), and these are
described above. There are in addition the following errors that are
specific to pcre_dfa_exec():
PCRE_ERROR_DFA_UITEM (-16)
This return is given if pcre_dfa_exec() encounters an item in the pat-
tern that it does not support, for instance, the use of \C or a back
reference.
PCRE_ERROR_DFA_UCOND (-17)
This return is given if pcre_dfa_exec() encounters a condition item
that uses a back reference for the condition, or a test for recursion
in a specific group. These are not supported.
PCRE_ERROR_DFA_UMLIMIT (-18)
This return is given if pcre_dfa_exec() is called with an extra block
that contains a setting of the match_limit field. This is not supported
(it is meaningless).
PCRE_ERROR_DFA_WSSIZE (-19)
This return is given if pcre_dfa_exec() runs out of space in the
workspace vector.
PCRE_ERROR_DFA_RECURSE (-20)
When a recursive subpattern is processed, the matching function calls
itself recursively, using private vectors for ovector and workspace.
This error is given if the output vector is not large enough. This
should be extremely rare, as a vector of size 1000 is used.
SEE ALSO
pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 24 August 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRECALLOUT(3) PCRECALLOUT(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE CALLOUTS
int (*pcre_callout)(pcre_callout_block *);
PCRE provides a feature called "callout", which is a means of temporar-
ily passing control to the caller of PCRE in the middle of pattern
matching. The caller of PCRE provides an external function by putting
its entry point in the global variable pcre_callout. By default, this
variable contains NULL, which disables all calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. Different callout points can be
identified by putting a number less than 256 after the letter C. The
default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
called, PCRE automatically inserts callouts, all with number 255,
before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
used with the pattern
A(\d{2}|--)
it is processed as if it were
(?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
Notice that there is a callout before and after each parenthesis and
alternation bar. Automatic callouts can be used for tracking the
progress of pattern matching. The pcretest command has an option that
sets automatic callouts; when it is used, the output indicates how the
pattern is matched. This is useful information when you are trying to
optimize the performance of a particular pattern.
MISSING CALLOUTS
You should be aware that, because of optimizations in the way PCRE
matches patterns, callouts sometimes do not happen. For example, if the
pattern is
ab(?C4)cd
PCRE knows that any matching string must contain the letter "d". If the
subject string is "abyz", the lack of "d" means that matching doesn't
ever start, and the callout is never reached. However, with "abyd",
though the result is still no match, the callout is obeyed.
THE CALLOUT INTERFACE
During matching, when PCRE reaches a callout point, the external func-
tion defined by pcre_callout is called (if it is set). This applies to
both the pcre_exec() and the pcre_dfa_exec() matching functions. The
only argument to the callout function is a pointer to a pcre_callout
block. This structure contains the following fields:
int version;
int callout_number;
int *offset_vector;
const char *subject;
int subject_length;
int start_match;
int current_position;
int capture_top;
int capture_last;
void *callout_data;
int pattern_position;
int next_item_length;
The version field is an integer containing the version number of the
block format. The initial version was 0; the current version is 1. The
version number will change again in future if additional fields are
added, but the intention is never to remove any of the existing fields.
The callout_number field contains the number of the callout, as com-
piled into the pattern (that is, the number after ?C for manual call-
outs, and 255 for automatically generated callouts).
The offset_vector field is a pointer to the vector of offsets that was
passed by the caller to pcre_exec() or pcre_dfa_exec(). When
pcre_exec() is used, the contents can be inspected in order to extract
substrings that have been matched so far, in the same way as for
extracting substrings after a match has completed. For pcre_dfa_exec()
this field is not useful.
The subject and subject_length fields contain copies of the values that
were passed to pcre_exec().
The start_match field normally contains the offset within the subject
at which the current match attempt started. However, if the escape
sequence \K has been encountered, this value is changed to reflect the
modified starting point. If the pattern is not anchored, the callout
function may be called several times from the same point in the pattern
for different starting points in the subject.
The current_position field contains the offset within the subject of
the current match pointer.
When the pcre_exec() function is used, the capture_top field contains
one more than the number of the highest numbered captured substring so
far. If no substrings have been captured, the value of capture_top is
one. This is always the case when pcre_dfa_exec() is used, because it
does not support captured substrings.
The capture_last field contains the number of the most recently cap-
tured substring. If no substrings have been captured, its value is -1.
This is always the case when pcre_dfa_exec() is used.
The callout_data field contains a value that is passed to pcre_exec()
or pcre_dfa_exec() specifically so that it can be passed back in call-
outs. It is passed in the pcre_callout field of the pcre_extra data
structure. If no such data was passed, the value of callout_data in a
pcre_callout block is NULL. There is a description of the pcre_extra
structure in the pcreapi documentation.
The pattern_position field is present from version 1 of the pcre_call-
out structure. It contains the offset to the next item to be matched in
the pattern string.
The next_item_length field is present from version 1 of the pcre_call-
out structure. It contains the length of the next item to be matched in
the pattern string. When the callout immediately precedes an alterna-
tion bar, a closing parenthesis, or the end of the pattern, the length
is zero. When the callout precedes an opening parenthesis, the length
is that of the entire subpattern.
The pattern_position and next_item_length fields are intended to help
in distinguishing between different automatic callouts, which all have
the same callout number. However, they are set for all callouts.
RETURN VALUES
The external callout function returns an integer to PCRE. If the value
is zero, matching proceeds as normal. If the value is greater than
zero, matching fails at the current point, but the testing of other
matching possibilities goes ahead, just as if a lookahead assertion had
failed. If the value is less than zero, the match is abandoned, and
pcre_exec() (or pcre_dfa_exec()) returns the negative value.
Negative values should normally be chosen from the set of
PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
reserved for use by callout functions; it will never be used by PCRE
itself.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 29 May 2007
Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------
PCRECOMPAT(3) PCRECOMPAT(3)
NAME
PCRE - Perl-compatible regular expressions
DIFFERENCES BETWEEN PCRE AND PERL
This document describes the differences in the ways that PCRE and Perl
handle regular expressions. The differences described here are mainly
with respect to Perl 5.8, though PCRE versions 7.0 and later contain
some features that are expected to be in the forthcoming Perl 5.10.
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
of what it does have are given in the section on UTF-8 support in the
main pcre page.
2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
permits them, but they do not mean what you might think. For example,
(?!a){3} does not assert that the next three characters are not "a". It
just asserts that the next character is not "a" three times.
3. Capturing subpatterns that occur inside negative lookahead asser-
tions are counted, but their entries in the offsets vector are never
set. Perl sets its numerical variables from any such patterns that are
matched before the assertion fails to match something (thereby succeed-
ing), but only if the negative lookahead assertion contains just one
branch.
4. Though binary zero characters are supported in the subject string,
they are not allowed in a pattern string because it is passed as a nor-
mal C string, terminated by zero. The escape sequence \0 can be used in
the pattern to represent a binary zero.
5. The following Perl escape sequences are not supported: \l, \u, \L,
\U, and \N. In fact these are implemented by Perl's general string-han-
dling and are not part of its pattern matching engine. If any of these
are encountered by PCRE, an error is generated.
6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
is built with Unicode character property support. The properties that
can be tested with \p and \P are limited to the general category prop-
erties such as Lu and Nd, script names such as Greek or Han, and the
derived properties Any and L&.
7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
ters in between are treated as literals. This is slightly different
from Perl in that $ and @ are also handled as literals inside the
quotes. In Perl, they cause variable interpolation (but of course PCRE
does not have variables). Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This
is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
"callout" feature allows an external function to be called during pat-
tern matching. See the pcrecallout documentation for details.
9. Subpatterns that are called recursively or as "subroutines" are
always treated as atomic groups in PCRE. This is like Python, but
unlike Perl.
10. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE it is set to "b".
11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
(*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
the forms without an argument. PCRE does not support (*MARK). If
(*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
ture group; this is different to Perl.
12. PCRE provides some extensions to the Perl regular expression facil-
ities. Perl 5.10 will include new features that are not in earlier
versions, some of which (such as named parentheses) have been in PCRE
for some time. This list is with respect to Perl 5.10:
(a) Although lookbehind assertions must match fixed length strings,
each alternative branch of a lookbehind assertion can match a different
length of string. Perl requires them all to have the same length.
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
meta-character matches only at the very end of the string.
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
ignored. (Perl can be made to issue a warning.)
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
fiers is inverted, that is, by default they are not greedy, but if fol-
lowed by a question mark they are.
(e) PCRE_ANCHORED can be used at matching time to force a pattern to be
tried only at the first matching position in the subject string.
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
TURE options for pcre_exec() have no Perl equivalents.
(g) The \R escape sequence can be restricted to match only CR, LF, or
CRLF by the PCRE_BSR_ANYCRLF option.
(h) The callout facility is PCRE-specific.
(i) The partial matching facility is PCRE-specific.
(j) Patterns compiled by PCRE can be saved and re-used at a later time,
even on different hosts that have the other endianness.
(k) The alternative matching function (pcre_dfa_exec()) matches in a
different way and is not Perl-compatible.
(l) PCRE recognizes some special sequences such as (*CR) at the start
of a pattern that set overall options that cannot be changed within the
pattern.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 11 September 2007
Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------
PCREPATTERN(3) PCREPATTERN(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions that are supported
by PCRE are described in detail below. There is a quick-reference syn-
tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
semantics as closely as it can. PCRE also supports some alternative
regular expression syntax (which does not conflict with the Perl syn-
tax) in order to provide some compatibility with regular expressions in
Python, .NET, and Oniguruma.
Perl's regular expressions are described in its own documentation, and
regular expressions in general are covered in a number of books, some
of which have copious examples. Jeffrey Friedl's "Mastering Regular
Expressions", published by O'Reilly, covers regular expressions in
great detail. This description of PCRE's regular expressions is
intended as reference material.
The original operation of PCRE was on strings of one-byte characters.
However, there is now also support for UTF-8 character strings. To use
this, you must build PCRE to include UTF-8 support, and then call
pcre_compile() with the PCRE_UTF8 option. How this affects pattern
matching is mentioned in several places below. There is also a summary
of UTF-8 features in the section on UTF-8 support in the main pcre
page.
The remainder of this document discusses the patterns that are sup-
ported by PCRE when its main matching function, pcre_exec(), is used.
From release 6.0, PCRE offers a second matching function,
pcre_dfa_exec(), which matches using a different algorithm that is not
Perl-compatible. Some of the features discussed below are not available
when pcre_dfa_exec() is used. The advantages and disadvantages of the
alternative function, and how it differs from the normal function, are
discussed in the pcrematching page.
NEWLINE CONVENTIONS
PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (line-
feed) character, the two-character sequence CRLF, any of the three pre-
ceding, or any Unicode newline sequence. The pcreapi page has further
discussion about newlines, and shows how to set the newline convention
in the options arguments for the compiling and matching functions.
It is also possible to specify a newline convention by starting a pat-
tern string with one of the following five sequences:
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
These override the default and the options given to pcre_compile(). For
example, on a Unix system where LF is the default newline sequence, the
pattern
(*CR)a.b
changes the convention to CR. That pattern matches "a\nb" because LF is
no longer a newline. Note that these special settings, which are not
Perl-compatible, are recognized only at the very start of a pattern,
and that they must be in upper case. If more than one of them is
present, the last one is used.
The newline convention does not affect what the \R escape sequence
matches. By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the description of \R
in the section entitled "Newline sequences" below. A change of \R set-
ting can be combined with a change of newline convention.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern, and match the corresponding characters in the subject. As a
trivial example, the pattern
The quick brown fox
matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE_CASELESS option), letters are
matched independently of case. In UTF-8 mode, PCRE always understands
the concept of case for characters whose values are less than 128, so
caseless matching is always possible. For characters with higher val-
ues, the concept of case is supported if PCRE is compiled with Unicode
property support, but not otherwise. If you want to use caseless
matching for characters 128 and above, you must ensure that PCRE is
compiled with Unicode property support as well as with UTF-8 support.
The power of regular expressions comes from the ability to include
alternatives and repetitions in the pattern. These are encoded in the
pattern by the use of metacharacters, which do not stand for themselves
but instead are interpreted in some special way.
There are two different sets of metacharacters: those that are recog-
nized anywhere in the pattern except within square brackets, and those
that are recognized within square brackets. Outside square brackets,
the metacharacters are as follows:
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character
class". In a character class the only metacharacters are:
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (only if followed by POSIX
syntax)
] terminates the character class
The following sections describe the use of each of the metacharacters.
BACKSLASH
The backslash character has several uses. Firstly, if it is followed by
a non-alphanumeric character, it takes away any special meaning that
character may have. This use of backslash as an escape character
applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the
pattern. This escaping action applies whether or not the following
character would otherwise be interpreted as a metacharacter, so it is
always safe to precede a non-alphanumeric with backslash to specify
that it stands for itself. In particular, if you want to match a back-
slash, you write \\.
If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
the pattern (other than in a character class) and characters between a
# outside a character class and the next newline are ignored. An escap-
ing backslash can be used to include a whitespace or # character as
part of the pattern.
If you want to remove the special meaning from a sequence of charac-
ters, you can do so by putting them between \Q and \E. This is differ-
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
tion. Note the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the
contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
Non-printing characters
A second use of backslash provides a way of encoding non-printing char-
acters in patterns in a visible manner. There is no restriction on the
appearance of non-printing characters, apart from the binary zero that
terminates a pattern, but when a pattern is being prepared by text
editing, it is usually easier to use one of the following escape
sequences than the binary character it represents:
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n linefeed (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
The precise effect of \cx is as follows: if x is a lower case letter,
it is converted to upper case. Then bit 6 of the character (hex 40) is
inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
becomes hex 7B.
After \x, from zero to two hexadecimal digits are read (letters can be
in upper or lower case). Any number of hexadecimal digits may appear
between \x{ and }, but the value of the character code must be less
than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
than the largest Unicode code point, which is 10FFFF.
If characters other than hexadecimal digits appear between \x{ and },
or if there is no terminating }, this form of escape is not recognized.
Instead, the initial \x will be interpreted as a basic hexadecimal
escape, with no following digits, giving a character whose value is
zero.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x. There is no difference in the way they are han-
dled. For example, \xdc is exactly the same as \x{dc}.
After \0 up to two further octal digits are read. If there are fewer
than two digits, just those that are present are used. Thus the
sequence \0\x\07 specifies two binary zeros followed by a BEL character
(code value 7). Make sure you supply two digits after the initial zero
if the pattern character that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is compli-
cated. Outside a character class, PCRE reads it and any following dig-
its as a decimal number. If the number is less than 10, or if there
have been at least that many previous capturing left parentheses in the
expression, the entire sequence is taken as a back reference. A
description of how this works is given later, following the discussion
of parenthesized subpatterns.
Inside a character class, or if the decimal number is greater than 9
and there have not been that many capturing subpatterns, PCRE re-reads
up to three octal digits following the backslash, and uses them to gen-
erate a data character. Any subsequent digits stand for themselves. In
non-UTF-8 mode, the value of a character specified in octal must be
less than \400. In UTF-8 mode, values up to \777 are permitted. For
example:
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
Note that octal values of 100 or greater must not be introduced by a
leading zero, because no more than three octal digits are ever read.
All the sequences that define a single character value can be used both
inside and outside character classes. In addition, inside a character
class, the sequence \b is interpreted as the backspace character (hex
08), and the sequences \R and \X are interpreted as the characters "R"
and "X", respectively. Outside a character class, these sequences have
different meanings (see below).
Absolute and relative back references
The sequence \g followed by an unsigned or a negative number, option-
ally enclosed in braces, is an absolute or relative back reference. A
named back reference can be coded as \g{name}. Back references are dis-
cussed later, following the discussion of parenthesized subpatterns.
Absolute and relative subroutine calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
an alternative syntax for referencing a subpattern as a "subroutine".
Details are discussed later. Note that \g{...} (Perl syntax) and
\g<...> (Oniguruma syntax) are not synonymous. The former is a back
reference; the latter is a subroutine call.
Generic character types
Another use of backslash is for specifying generic character types. The
following are always recognized:
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal whitespace character
\H any character that is not a horizontal whitespace character
\s any whitespace character
\S any character that is not a whitespace character
\v any vertical whitespace character
\V any character that is not a vertical whitespace character
\w any "word" character
\W any "non-word" character
Each pair of escape sequences partitions the complete set of characters
into two disjoint sets. Any given character matches one, and only one,
of each pair.
These character type sequences can appear both inside and outside char-
acter classes. They each match one character of the appropriate type.
If the current matching point is at the end of the subject string, all
of them fail, since there is no character to match.
For compatibility with Perl, \s does not match the VT character (code
11). This makes it different from the the POSIX "space" class. The \s
characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
"use locale;" is included in a Perl script, \s may match the VT charac-
ter. In PCRE, it never does.
In UTF-8 mode, characters with values greater than 128 never match \d,
\s, or \w, and always match \D, \S, and \W. This is true even when Uni-
code character property support is available. These sequences retain
their original meanings from before UTF-8 support was available, mainly
for efficiency reasons.
The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
the other sequences, these do match certain high-valued codepoints in
UTF-8 mode. The horizontal space characters are:
U+0009 Horizontal tab
U+0020 Space
U+00A0 Non-break space
U+1680 Ogham space mark
U+180E Mongolian vowel separator
U+2000 En quad
U+2001 Em quad
U+2002 En space
U+2003 Em space
U+2004 Three-per-em space
U+2005 Four-per-em space
U+2006 Six-per-em space
U+2007 Figure space
U+2008 Punctuation space
U+2009 Thin space
U+200A Hair space
U+202F Narrow no-break space
U+205F Medium mathematical space
U+3000 Ideographic space
The vertical space characters are:
U+000A Linefeed
U+000B Vertical tab
U+000C Formfeed
U+000D Carriage return
U+0085 Next line
U+2028 Line separator
U+2029 Paragraph separator
A "word" character is an underscore or any character less than 256 that
is a letter or digit. The definition of letters and digits is con-
trolled by PCRE's low-valued character tables, and may vary if locale-
specific matching is taking place (see "Locale support" in the pcreapi
page). For example, in a French locale such as "fr_FR" in Unix-like
systems, or "french" in Windows, some character codes greater than 128
are used for accented letters, and these are matched by \w. The use of
locales with Unicode is discouraged.
Newline sequences
Outside a character class, by default, the escape sequence \R matches
any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
mode \R is equivalent to the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details of which are given
below. This particular group matches either the two-character sequence
CR followed by LF, or one of the single characters LF (linefeed,
U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
return, U+000D), or NEL (next line, U+0085). The two-character sequence
is treated as a single unit that cannot be split.
In UTF-8 mode, two additional characters whose codepoints are greater
than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
rator, U+2029). Unicode character property support is not needed for
these characters to be recognized.
It is possible to restrict \R to match only CR, LF, or CRLF (instead of
the complete set of Unicode line endings) by setting the option
PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
(BSR is an abbrevation for "backslash R".) This can be made the default
when PCRE is built; if this is the case, the other behaviour can be
requested via the PCRE_BSR_UNICODE option. It is also possible to
specify these settings by starting a pattern string with one of the
following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
These override the default and the options given to pcre_compile(), but
they can be overridden by options given to pcre_exec(). Note that these
special settings, which are not Perl-compatible, are recognized only at
the very start of a pattern, and that they must be in upper case. If
more than one of them is present, the last one is used. They can be
combined with a change of newline convention, for example, a pattern
can start with:
(*ANY)(*BSR_ANYCRLF)
Inside a character class, \R matches the letter "R".
Unicode character properties
When PCRE is built with Unicode character property support, three addi-
tional escape sequences that match characters with specific properties
are available. When not in UTF-8 mode, these sequences are of course
limited to testing characters whose codepoints are less than 256, but
they do work in this mode. The extra escape sequences are:
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\X an extended Unicode sequence
The property names represented by xx above are limited to the Unicode
script names, the general category properties, and "Any", which matches
any character (including newline). Other properties such as "InMusical-
Symbols" are not currently supported by PCRE. Note that \P{Any} does
not match any characters, so always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts.
A character from one of these sets can be matched using a script name.
For example:
\p{Greek}
\P{Han}
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
Each character has exactly one general category property, specified by
a two-letter abbreviation. For compatibility with Perl, negation can be
specified by including a circumflex between the opening brace and the
property name. For example, \p{^Lu} is the same as \P{Lu}.
If only one letter is specified with \p or \P, it includes all the gen-
eral category properties that start with that letter. In this case, in
the absence of negation, the curly brackets in the escape sequence are
optional; these two examples have the same effect:
\p{L}
\pL
The following general category property codes are supported:
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
The special property L& is also supported: it matches a character that
has the Lu, Ll, or Lt property, in other words, a letter that is not
classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range
U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
the pcreapi page).
The long synonyms for these properties that Perl supports (such as
\p{Letter}) are not supported by PCRE, nor is it permitted to prefix
any of these properties with "Is".
No character that is in the Unicode table has the Cn (unassigned) prop-
erty. Instead, this property is assumed for any code point that is not
in the Unicode table.
Specifying caseless matching does not affect these escape sequences.
For example, \p{Lu} always matches only upper case letters.
The \X escape matches any number of Unicode characters that form an
extended Unicode sequence. \X is equivalent to
(?>\PM\pM*)
That is, it matches a character without the "mark" property, followed
by zero or more characters with the "mark" property, and treats the
sequence as an atomic group (see below). Characters with the "mark"
property are typically accents that affect the preceding character.
None of them have codepoints less than 256, so in non-UTF-8 mode \X
matches any one character.
Matching characters by Unicode property is not fast, because PCRE has
to search a structure that contains data for over fifteen thousand
characters. That is why the traditional escape sequences such as \d and
\w do not use Unicode properties in PCRE.
Resetting the match start
The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
ously matched characters not to be included in the final matched
sequence. For example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar". This feature
is similar to a lookbehind assertion (described below). However, in
this case, the part of the subject before the real match does not have
to be of fixed length, as lookbehind assertions do. The use of \K does
not interfere with the setting of captured substrings. For example,
when the pattern
(foo)\Kbar
matches "foobar", the first substring is still set to "foo".
Simple assertions
The final use of backslash is for certain simple assertions. An asser-
tion specifies a condition that has to be met at a particular point in
a match, without consuming any characters from the subject string. The
use of subpatterns for more complicated assertions is described below.
The backslashed assertions are:
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject
These assertions may not appear in character classes (but note that \b
has a different meaning, namely the backspace character, inside a char-
acter class).
A word boundary is a position in the subject string where the current
character and the previous character do not both match \w or \W (i.e.
one matches \w and the other matches \W), or the start or end of the
string if the first or last character matches \w, respectively.
The \A, \Z, and \z assertions differ from the traditional circumflex
and dollar (described in the next section) in that they only ever match
at the very start and end of the subject string, whatever options are
set. Thus, they are independent of multiline mode. These three asser-
tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
affect only the behaviour of the circumflex and dollar metacharacters.
However, if the startoffset argument of pcre_exec() is non-zero, indi-
cating that matching is to start at a point other than the beginning of
the subject, \A can never match. The difference between \Z and \z is
that \Z matches before a newline at the end of the string as well as at
the very end, whereas \z matches only at the end.
The \G assertion is true only when the current matching position is at
the start point of the match, as specified by the startoffset argument
of pcre_exec(). It differs from \A when the value of startoffset is
non-zero. By calling pcre_exec() multiple times with appropriate argu-
ments, you can mimic Perl's /g option, and it is in this kind of imple-
mentation where \G can be useful.
Note, however, that PCRE's interpretation of \G, as the start of the
current match, is subtly different from Perl's, which defines it as the
end of the previous match. In Perl, these can be different when the
previously matched string was empty. Because PCRE does just one match
at a time, it cannot reproduce this behaviour.
If all the alternatives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored" flag is set
in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching
point is at the start of the subject string. If the startoffset argu-
ment of pcre_exec() is non-zero, circumflex can never match if the
PCRE_MULTILINE option is unset. Inside a character class, circumflex
has an entirely different meaning (see below).
Circumflex need not be the first character of the pattern if a number
of alternatives are involved, but it should be the first thing in each
alternative in which it appears if the pattern is ever to match that
branch. If all possible alternatives start with a circumflex, that is,
if the pattern is constrained to match only at the start of the sub-
ject, it is said to be an "anchored" pattern. (There are also other
constructs that can cause a pattern to be anchored.)
A dollar character is an assertion that is true only if the current
matching point is at the end of the subject string, or immediately
before a newline at the end of the string (by default). Dollar need not
be the last character of the pattern if a number of alternatives are
involved, but it should be the last item in any branch in which it
appears. Dollar has no special meaning in a character class.
The meaning of dollar can be changed so that it matches only at the
very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, a circumflex
matches immediately after internal newlines as well as at the start of
the subject string. It does not match after a newline that ends the
string. A dollar matches before any newlines in the string, as well as
at the very end, when PCRE_MULTILINE is set. When newline is specified
as the two-character sequence CRLF, isolated CR and LF characters do
not indicate newlines.
For example, the pattern /^abc$/ matches the subject string "def\nabc"
(where \n represents a newline) in multiline mode, but not otherwise.
Consequently, patterns that are anchored in single line mode because
all branches start with ^ are not anchored in multiline mode, and a
match for circumflex is possible when the startoffset argument of
pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
PCRE_MULTILINE is set.
Note that the sequences \A, \Z, and \z can be used to match the start
and end of the subject in both modes, and if all branches of a pattern
start with \A it is always anchored, whether or not PCRE_MULTILINE is
set.
FULL STOP (PERIOD, DOT)
Outside a character class, a dot in the pattern matches any one charac-
ter in the subject string except (by default) a character that signi-
fies the end of a line. In UTF-8 mode, the matched character may be
more than one byte long.
When a line ending is defined as a single character, dot never matches
that character; when the two-character sequence CRLF is used, dot does
not match CR if it is immediately followed by LF, but otherwise it
matches all characters (including isolated CRs and LFs). When any Uni-
code line endings are being recognized, dot does not match CR or LF or
any of the other line ending characters.
The behaviour of dot with regard to newlines can be changed. If the
PCRE_DOTALL option is set, a dot matches any one character, without
exception. If the two-character sequence CRLF is present in the subject
string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circum-
flex and dollar, the only relationship being that they both involve
newlines. Dot has no special meaning in a character class.
MATCHING A SINGLE BYTE
Outside a character class, the escape sequence \C matches any one byte,
both in and out of UTF-8 mode. Unlike a dot, it always matches any
line-ending characters. The feature is provided in Perl in order to
match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
acters into individual bytes, what remains in the string may be a mal-
formed UTF-8 string. For this reason, the \C escape sequence is best
avoided.
PCRE does not allow \C to appear in lookbehind assertions (described
below), because in UTF-8 mode this would make it impossible to calcu-
late the length of the lookbehind.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not spe-
cial. If a closing square bracket is required as a member of the class,
it should be the first data character in the class (after an initial
circumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject. In UTF-8
mode, the character may occupy more than one byte. A matched character
must be in the set of characters defined by the class, unless the first
character in the class definition is a circumflex, in which case the
subject character must not be in the set defined by the class. If a
circumflex is actually required as a member of the class, ensure it is
not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lower case vowel,
while [^aeiou] matches any character that is not a lower case vowel.
Note that a circumflex is just a convenient notation for specifying the
characters that are in the class by enumerating those that are not. A
class that starts with a circumflex is not an assertion: it still con-
sumes a character from the subject string, and therefore it fails if
the current pointer is at the end of the string.
In UTF-8 mode, characters with values greater than 255 can be included
in a class as a literal string of bytes, or by using the \x{ escaping
mechanism.
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, so for example, a caseless
[aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
understands the concept of case for characters whose values are less
than 128, so caseless matching is always possible. For characters with
higher values, the concept of case is supported if PCRE is compiled
with Unicode property support, but not otherwise. If you want to use
caseless matching for characters 128 and above, you must ensure that
PCRE is compiled with Unicode property support as well as with UTF-8
support.
Characters that might indicate line breaks are never treated in any
special way when matching character classes, whatever line-ending
sequence is in use, and whatever setting of the PCRE_DOTALL and
PCRE_MULTILINE options is used. A class such as [^a] always matches one
of these characters.
The minus (hyphen) character can be used to specify a range of charac-
ters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
It is not possible to have the literal character "]" as the end charac-
ter of a range. A pattern such as [W-]46] is interpreted as a class of
two characters ("W" and "-") followed by a literal string "46]", so it
would match "W46]" or "-46]". However, if the "]" is escaped with a
backslash it is interpreted as the end of range, so [W-\]46] is inter-
preted as a class containing a range followed by two other characters.
The octal or hexadecimal representation of "]" can also be used to end
a range.
Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example
[\000-\037]. In UTF-8 mode, ranges can include characters whose values
are greater than 255, for example [\x{100}-\x{2ff}].
If a range that includes letters is used when caseless matching is set,
it matches the letters in either case. For example, [W-c] is equivalent
to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
character tables for a French locale are in use, [\xc8-\xcb] matches
accented E characters in both cases. In UTF-8 mode, PCRE supports the
concept of case for characters with values greater than 128 only when
it is compiled with Unicode property support.
The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
in a character class, and add the characters that they match to the
class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
flex can conveniently be used with the upper case character types to
specify a more restricted set of characters than the matching lower
case type. For example, the class [^\W_] matches any letter or digit,
but not underscore.
The only metacharacters that are recognized in character classes are
backslash, hyphen (only where it can be interpreted as specifying a
range), circumflex (only at the start), opening square bracket (only
when it can be interpreted as introducing a POSIX class name - see the
next section), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example,
[01[:alpha:]%]
matches "0", "1", any alphabetic character, or "%". The supported class
names are
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits
space white space (not quite the same as \s)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). Notice that this list includes the VT character (code
11). This makes "space" different to \s, which does not include VT (for
Perl compatibility).
The name "word" is a Perl extension, and "blank" is a GNU extension
from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example,
[12[:^digit:]]
matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
these are not supported, and an error is given if they are encountered.
In UTF-8 mode, characters with values greater than 128 do not match any
of the POSIX character classes.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For
example, the pattern
gilbert|sullivan
matches either "gilbert" or "sullivan". Any number of alternatives may
appear, and an empty alternative is permitted (matching the empty
string). The matching process tries each alternative in turn, from left
to right, and the first one that succeeds is used. If the alternatives
are within a subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
PCRE_EXTENDED options (which are Perl-compatible) can be changed from
within the pattern by a sequence of Perl option letters enclosed
between "(?" and ")". The option letters are
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
is also permitted. If a letter appears both before and after the
hyphen, the option is unset.
The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
can be changed in the same way as the Perl-compatible options by using
the characters J, U and X respectively.
When an option change occurs at top level (that is, not inside subpat-
tern parentheses), the change applies to the remainder of the pattern
that follows. If the change is placed right at the start of a pattern,
PCRE extracts it into the global options (and it will therefore show up
in data extracted by the pcre_fullinfo() function).
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the current pattern that follows
it, so
(a(?i)b)c
matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
used). By this means, options can be made to have different settings
in different parts of the pattern. Any changes made in one alternative
do carry on into subsequent branches within the same subpattern. For
example,
(a(?i)b|c)
matches "ab", "aB", "c", and "C", even though when matching "C" the
first branch is abandoned before the option setting. This is because
the effects of option settings happen at compile time. There would be
some very weird behaviour otherwise.
Note: There are other PCRE-specific options that can be set by the
application when the compile or match functions are called. In some
cases the pattern can contain special leading sequences to override
what the application has set or what has been defaulted. Details are
given in the section entitled "Newline sequences" above.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern does two things:
1. It localizes a set of alternatives. For example, the pattern
cat(aract|erpillar|)
matches one of the words "cat", "cataract", or "caterpillar". Without
the parentheses, it would match "cataract", "erpillar" or an empty
string.
2. It sets up the subpattern as a capturing subpattern. This means
that, when the whole pattern matches, that portion of the subject
string that matched the subpattern is passed back to the caller via the
ovector argument of pcre_exec(). Opening parentheses are counted from
left to right (starting from 1) to obtain numbers for the capturing
subpatterns.
For example, if the string "the red king" is matched against the pat-
tern
the ((red|white) (king|queen))
the captured substrings are "red king", "red", and "king", and are num-
bered 1, 2, and 3, respectively.
The fact that plain parentheses fulfil two functions is not always
helpful. There are often times when a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any captur-
ing, and is not counted when computing the number of any subsequent
capturing subpatterns. For example, if the string "the white queen" is
matched against the pattern
the ((?:red|white) (king|queen))
the captured substrings are "white queen" and "queen", and are numbered
1 and 2. The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters may appear
between the "?" and the ":". Thus the two patterns
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
match exactly the same set of strings. Because alternative branches are
tried from left to right, and options are not reset until the end of
the subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
uses the same numbers for its capturing parentheses. Such a subpattern
starts with (?| and is itself a non-capturing subpattern. For example,
consider this pattern:
(?|(Sat)ur|(Sun))day
Because the two alternatives are inside a (?| group, both sets of cap-
turing parentheses are numbered one. Thus, when the pattern matches,
you can look at captured substring number one, whichever alternative
matched. This construct is useful when you want to capture part, but
not all, of one of a number of alternatives. Inside a (?| group, paren-
theses are numbered as usual, but the number is reset at the start of
each branch. The numbers of any capturing buffers that follow the sub-
pattern start after the highest number used in any branch. The follow-
ing example is taken from the Perl documentation. The numbers under-
neath show in which buffer the captured content will be stored.
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
A backreference or a recursive call to a numbered subpattern always
refers to the first one in the pattern with the given number.
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres-
sions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE supports the naming of sub-
patterns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE introduced it at release 4.0, using
the Python syntax. PCRE now supports both the Perl and the Python syn-
tax.
In PCRE, a subpattern can be named in one of three ways: (?<name>...)
or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
to capturing parentheses from other parts of the pattern, such as back-
references, recursion, and conditions, can be made by name as well as
by number.
Names consist of up to 32 alphanumeric characters and underscores.
Named capturing parentheses are still allocated numbers as well as
names, exactly as if the names were not present. The PCRE API provides
function calls for extracting the name-to-number translation table from
a compiled pattern. There is also a convenience function for extracting
a captured substring by name.
By default, a name must be unique within a pattern, but it is possible
to relax this constraint by setting the PCRE_DUPNAMES option at compile
time. This can be useful for patterns where only one instance of the
named parentheses can match. Suppose you want to match the name of a
weekday, either as a 3-letter abbreviation or as the full name, and in
both cases you want to extract the abbreviation. This pattern (ignoring
the line breaks) does the job:
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
There are five capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a "branch
reset" subpattern, as described in the previous section.)
The convenience function for extracting the data by name returns the
substring for the first (and in this example, the only) subpattern of
that name that matched. This saves searching to find which numbered
subpattern it was. If you make a reference to a non-unique named sub-
pattern from elsewhere in the pattern, the one that corresponds to the
lowest number is used. For further details of the interfaces for han-
dling named subpatterns, see the pcreapi documentation.
REPETITION
Repetition is specified by quantifiers, which can follow any of the
following items:
a literal data character
the dot metacharacter
the \C escape sequence
the \X escape sequence (in UTF-8 mode with Unicode properties)
the \R escape sequence
an escape such as \d that matches a single character
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion)
The general repetition quantifier specifies a minimum and maximum num-
ber of permitted matches, by giving the two numbers in curly brackets
(braces), separated by a comma. The numbers must be less than 65536,
and the first must be less than or equal to the second. For example:
z{2,4}
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
special character. If the second number is omitted, but the comma is
present, there is no upper limit; if the second number and the comma
are both omitted, the quantifier specifies an exact number of required
matches. Thus
[aeiou]{3,}
matches at least 3 successive vowels, but may match many more, while
\d{8}
matches exactly 8 digits. An opening curly bracket that appears in a
position where a quantifier is not allowed, or one that does not match
the syntax of a quantifier, is taken as a literal character. For exam-
ple, {,6} is not a quantifier, but a literal string of four characters.
In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
acters, each of which is represented by a two-byte sequence. Similarly,
when Unicode property support is available, \X{3} matches three Unicode
extended sequences, each of which may be several bytes long (and they
may be of different lengths).
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present. This may be use-
ful for subpatterns that are referenced as subroutines from elsewhere
in the pattern. Items other than subpatterns that have a {0} quantifier
are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-charac-
ter abbreviations:
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
It is possible to construct infinite loops by following a subpattern
that can match no characters with a quantifier that has no upper limit,
for example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, because there are cases where this can be
useful, such patterns are now accepted, but if any repetition of the
subpattern does in fact match no characters, the loop is forcibly bro-
ken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the rest of the pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between /* and */ and within the comment, individual * and /
characters may appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, because it matches the entire string owing to the greediness of
the .* item.
However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the pattern
/\*.*?\*/
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of
matches. Do not confuse this use of question mark with its use as a
quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the
only way the rest of the pattern matches.
If the PCRE_UNGREEDY option is set (an option that is not available in
Perl), the quantifiers are not greedy by default, but individual ones
can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
When a parenthesized subpattern is quantified with a minimum repeat
count that is greater than 1 or with a limited maximum, more memory is
required for the compiled pattern, in proportion to the size of the
minimum or maximum.
If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
alent to Perl's /s) is set, thus allowing the dot to match newlines,
the pattern is implicitly anchored, because whatever follows will be
tried against every character position in the subject string, so there
is no point in retrying the overall match at any position after the
first. PCRE normally treats such a pattern as though it were preceded
by \A.
In cases where it is known that the subject string contains no new-
lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
mization, or alternatively using ^ to indicate anchoring explicitly.
However, there is one situation where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a
backreference elsewhere in the pattern, a match at the start may fail
where a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
When a capturing subpattern is repeated, the value captured is the sub-
string that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
is "tweedledee". However, if there are nested capturing subpatterns,
the corresponding captured values may have been set in previous itera-
tions. For example, after
/(a|(b))+/
matches "aba" the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item
to be re-evaluated to see if a different number of repeats allows the
rest of the pattern to match. Sometimes it is useful to prevent this,
either to change the nature of the match, or to cause it fail earlier
than it otherwise might, when the author of the pattern knows there is
no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the
\d+ item, and then with 4, and so on, before ultimately failing.
"Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
the means for specifying that once a subpattern has matched, it is not
to be re-evaluated in this way.
If we use atomic grouping for the previous example, the matcher gives
up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it con-
tains once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+? are pre-
pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above, a
simpler notation, called a "possessive quantifier" can be used. This
consists of an additional + character following a quantifier. Using
this notation, the previous example can be rewritten as
\d++foo
Note that a possessive quantifier can be used with an entire group, for
example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of the
PCRE_UNGREEDY option is ignored. They are a convenient notation for the
simpler forms of atomic group. However, there is no difference in the
meaning of a possessive quantifier and the equivalent atomic group,
though there may be a performance difference; possessive quantifiers
should be slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syn-
tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
built Sun's Java package, and PCRE copied it from there. It ultimately
found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain sim-
ple pattern constructs. For example, the sequence A+B is treated as
A++B because there is no point in backtracking into a sequence of A's
when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
very long time indeed. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the external
* repeat in a large number of ways, and all have to be tried. (The
example uses [!?] rather than a single character at the end, because
both PCRE and Perl have an optimization that allows for fast failure
when a single character is used. They remember the last single charac-
ter that is required for a match, and fail early if it is not present
in the string.) If the pattern is changed so that it uses an atomic
group, like this:
((?>\D+)|<\d+>)*[!?]
sequences of non-digits cannot be broken, and failure happens quickly.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub-
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 10,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pat-
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 10. A "forward back
reference" of this type can make sense when a repetition is involved
and the subpattern to the right has participated in an earlier itera-
tion.
It is not possible to have a numerical "forward back reference" to a
subpattern whose number is 10 or more using this syntax because a
sequence such as \50 is interpreted as a character defined in octal.
See the subsection entitled "Non-printing characters" above for further
details of the handling of digits following a backslash. There is no
such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
Another way of avoiding the ambiguity inherent in the use of digits
following a backslash is to use the \g escape sequence, which is a fea-
ture introduced in Perl 5.10. This escape must be followed by an
unsigned number or a negative number, optionally enclosed in braces.
These examples are all identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the ambigu-
ity that is present in the older syntax. It is also useful when literal
digits follow the reference. A negative number is a relative reference.
Consider this example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started captur-
ing subpattern before \g, that is, is it equivalent to \2. Similarly,
\g{-2} would be equivalent to \1. The use of relative references can be
helpful in long patterns, and also in patterns that are created by
joining together fragments that contain references within themselves.
A back reference matches whatever actually matched the capturing sub-
pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If caseful matching is in force at the
time of the back reference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capturing subpattern is matched caselessly.
There are several different ways of writing back references to named
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
\k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
unified back reference syntax, in which \g can be used for both numeric
and named references, is also supported. We could rewrite the above
example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name may appear in the pattern
before or after the reference.
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail. For example, the pattern
(a|(bc))\2
always fails if it starts to match "a" rather than "bc". Because there
may be many capturing parentheses in a pattern, all digits following
the backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be
used to terminate the back reference. If the PCRE_EXTENDED option is
set, this can be whitespace. Otherwise an empty comment (see "Com-
ments" below) can be used.
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the subpattern, the back reference matches the character
string corresponding to the previous iteration. In order for this to
work, the pattern must be such that the first iteration does not need
to match the back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
ASSERTIONS
An assertion is a test on the characters following or preceding the
current matching point that does not actually consume any characters.
The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
described above.
More complicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the subject
string, and those that look behind it. An assertion subpattern is
matched in the normal way, except that it does not cause the current
matching position to be changed.
Assertion subpatterns are not capturing subpatterns, and may not be
repeated, because it makes no sense to assert the same thing several
times. If any kind of assertion contains capturing subpatterns within
it, these are counted for the purposes of numbering the capturing sub-
patterns in the whole pattern. However, substring capturing is carried
out only for positive assertions, because it does not make sense for
negative assertions.
Lookahead assertions
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
\w+(?=;)
matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
most convenient way to do it is with (?!) because an empty string
always matches, so an assertion that requires there not to be an empty
string must always fail.
Lookbehind assertions
Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
does find an occurrence of "bar" that is not preceded by "foo". The
contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev-
eral top-level alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
is permitted, but
(?<!dogs?|cats?)
causes an error at compile time. Branches that match different length
strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl (at least for 5.8), which
requires all branches to match the same length of string. An assertion
such as
(?<=ab(c|de))
is not permitted, because its single top-level branch can match two
different lengths, but it is acceptable if rewritten to use two top-
level branches:
(?<=abc|abde)
In some cases, the Perl 5.10 escape sequence \K (see above) can be used
instead of a lookbehind assertion; this is not restricted to a fixed-
length.
The implementation of lookbehind assertions is, for each alternative,
to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the cur-
rent position, the assertion fails.
PCRE does not allow the \C escape (which matches a single byte in UTF-8
mode) to appear in lookbehind assertions, because it makes it impossi-
ble to calculate the length of the lookbehind. The \X and \R escapes,
which can match different numbers of bytes, are also not permitted.
Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching at the end of the subject
string. Consider a simple pattern such as
abcd$
when applied to a long string that does not match. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject
and then see if what follows matches the rest of the pattern. If the
pattern is specified as
^.*abcd$
the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
last character, then all but the last two characters, and so on. Once
again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
Using multiple assertions
Several assertions (of any sort) may occur in succession. For example,
(?<=\d{3})(?<!999)foo
matches "foo" preceded by three digits that are not "999". Notice that
each of the assertions is applied independently at the same point in
the subject string. First there is a check that the previous three
characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre-
ceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it doesn't match "123abc-
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example,
(?<=(?<!foo)bar)baz
matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern con-
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a previous capturing subpat-
tern matched or not. The two possible forms of conditional subpattern
are
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alterna-
tives in the subpattern, a compile-time error occurs.
There are four kinds of condition: references to subpatterns, refer-
ences to recursion, a pseudo-condition called DEFINE, and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of digits,
the condition is true if the capturing subpattern of that number has
previously matched. An alternative notation is to precede the digits
with a plus or minus sign. In this case, the subpattern number is rela-
tive rather than absolute. The most recently opened parentheses can be
referenced by (?(-1), the next most recent by (?(-2), and so on. In
looping constructs it can also make sense to refer to subsequent groups
with constructs such as (?(+2).
Consider the following pattern, which contains non-significant white
space to make it more readable (assume the PCRE_EXTENDED option) and to
divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec-
ond part matches one or more characters that are not parentheses. The
third part is a conditional subpattern that tests whether the first set
of parentheses matched or not. If they did, that is, if subject started
with an opening parenthesis, the condition is true, and so the yes-pat-
tern is executed and a closing parenthesis is required. Otherwise,
since no-pattern is not present, the subpattern matches nothing. In
other words, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If you were embedding this pattern in a larger one, you could use a
relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger
pattern.
Checking for a used subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern by name. For compatibility with earlier versions of
PCRE, which had this facility before Perl, the syntax (?(name)...) is
also recognized. However, there is a possible ambiguity with this syn-
tax, because subpattern names may consist entirely of digits. PCRE
looks first for a named subpattern; if it cannot find one and the name
consists entirely of digits, PCRE looks for a subpattern of that num-
ber, which must be greater than zero. Using subpattern names that con-
sist entirely of digits is not recommended.
Rewriting the above example to use a named subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
Checking for pattern recursion
If the condition is the string (R), and there is no subpattern with the
name R, the condition is true if a recursive call to the whole pattern
or any subpattern has been made. If digits or a name preceded by amper-
sand follow the letter R, for example:
(?(R3)...) or (?(R&name;)...)
the condition is true if the most recent recursion is into the subpat-
tern whose number or name is given. This condition does not check the
entire recursion stack.
At "top level", all these recursion test conditions are false. Recur-
sive patterns are described below.
Defining subpatterns for use by reference only
If the condition is the string (DEFINE), and there is no subpattern
with the name DEFINE, the condition is always false. In this case,
there may be only one alternative in the subpattern. It is always
skipped if control reaches this point in the pattern; the idea of
DEFINE is that it can be used to define "subroutines" that can be ref-
erenced from elsewhere. (The use of "subroutines" is described below.)
For example, a pattern to match an IPv4 address could be written like
this (ignore whitespace and line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte;) (\.(?&byte;)){3} \b
The first part of the pattern is a DEFINE group inside which a another
group named "byte" is defined. This matches an individual component of
an IPv4 address (a number less than 256). When matching takes place,
this part of the pattern is skipped because DEFINE acts like a false
condition.
The rest of the pattern uses references to the named group to match the
four dot-separated components of an IPv4 address, insisting on a word
boundary at each end.
Assertion conditions
If the condition is not in any of the above formats, it must be an
assertion. This may be a positive or negative lookahead or lookbehind
assertion. Consider this pattern, again containing non-significant
white space, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. In other words,
it tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative;
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
COMMENTS
The sequence (?# marks the start of a comment that continues up to the
next closing parenthesis. Nested parentheses are not permitted. The
characters that make up a comment play no part in the pattern matching
at all.
If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues to immediately
after the next newline in the pattern.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular expres-
sions to recurse (amongst other things). It does this by interpolating
Perl code in the expression at run time, and the code can refer to the
expression itself. A Perl pattern using code interpolation to solve the
parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
The (?p{...}) item interpolates Perl code at run time, and in this case
refers recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead,
it supports special syntax for recursion of the entire pattern, and
also for individual subpattern recursion. After its introduction in
PCRE and Python, this kind of recursion was introduced into Perl at
release 5.10.
A special item that consists of (? followed by a number greater than
zero and a closing parenthesis is a recursive call of the subpattern of
the given number, provided that it occurs inside that subpattern. (If
not, it is a "subroutine" call, which is described in the next sec-
tion.) The special item (?R) or (?0) is a recursive call of the entire
regular expression.
In PCRE (like Python, but unlike Perl), a recursive subpattern call is
always treated as an atomic group. That is, once it has matched some of
the subject string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure.
This PCRE pattern solves the nested parentheses problem (assume the
PCRE_EXTENDED option is set so that white space is ignored):
\( ( (?>[^()]+) | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a
recursive match of the pattern itself (that is, a correctly parenthe-
sized substring). Finally there is a closing parenthesis.
If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
( \( ( (?>[^()]+) | (?1) )* \) )
We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references. (A Perl
5.10 feature.) Instead of (?1) in the pattern above you can write
(?-2) to refer to the second most recently opened parentheses preceding
the recursion. In other words, a negative number counts capturing
parentheses leftwards from the point at which it is encountered.
It is also possible to refer to subsequently opened parentheses, by
writing references such as (?+2). However, these cannot be recursive
because the reference is not inside the parentheses that are refer-
enced. They are always "subroutine" calls, as described in the next
section.
An alternative approach is to use named parentheses instead. The Perl
syntax for this is (?&name;); PCRE's earlier syntax (?P>name) is also
supported. We could rewrite the above example as follows:
(?<pn> \( ( (?>[^()]+) | (?&pn;) )* \) )
If there is more than one subpattern with the same name, the earliest
one is used.
This particular example pattern that we have been looking at contains
nested unlimited repeats, and so the use of atomic grouping for match-
ing strings of non-parentheses is important when applying the pattern
to strings that do not match. For example, when this pattern is applied
to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it yields "no match" quickly. However, if atomic grouping is not used,
the match runs for a very long time indeed because there are so many
different ways the + and * repeats can carve up the subject, and all
have to be tested before failure can be reported.
At the end of a match, the values set for any capturing subpatterns are
those from the outermost level of the recursion at which the subpattern
value is set. If you want to obtain intermediate values, a callout
function can be used (see below and the pcrecallout documentation). If
the pattern above is matched against
(ab(cd)ef)
the value for the capturing parentheses is "ef", which is the last
value taken on at the top level. If additional parentheses are added,
giving
\( ( ( (?>[^()]+) | (?R) )* ) \)
^ ^
^ ^
the string they capture is "ab(cd)ef", the contents of the top level
parentheses. If there are more than 15 capturing parentheses in a pat-
tern, PCRE has to obtain extra memory to store data during a recursion,
which it does by using pcre_malloc, freeing it via pcre_free after-
wards. If no memory can be obtained, the match fails with the
PCRE_ERROR_NOMEMORY error.
Do not confuse the (?R) item with the condition (R), which tests for
recursion. Consider this pattern, which matches text in angle brack-
ets, allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), whereas any characters are permit-
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
In this pattern, (?(R) is the start of a conditional subpattern, with
two different alternatives for the recursive and non-recursive cases.
The (?R) item is the actual recursive call.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern reference (either by number or
by name) is used outside the parentheses to which it refers, it oper-
ates like a subroutine in a programming language. The "called" subpat-
tern may be defined before or after the reference. A numbered reference
can be absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
An earlier example pointed out that the pattern
(sens|respons)e and \1ibility
matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
is used, it does match "sense and responsibility" as well as the other
two strings. Another example is given in the discussion of DEFINE
above.
Like recursive subpatterns, a "subroutine" call is always treated as an
atomic group. That is, once it has matched some of the subject string,
it is never re-entered, even if it contains untried alternatives and
there is a subsequent matching failure.
When a subpattern is used as a subroutine, processing options such as
case-independence are fixed when the subpattern is defined. They cannot
be changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
an alternative syntax for referencing a subpattern as a subroutine,
possibly recursively. Here are two of the examples used above, rewrit-
ten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
(abc)(?i:\g<-1>)
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
synonymous. The former is a back reference; the latter is a subroutine
call.
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
PCRE provides a similar feature, but of course it cannot obey arbitrary
Perl code. The feature is called "callout". The caller of PCRE provides
an external function by putting its entry point in the global variable
pcre_callout. By default, this variable contains NULL, which disables
all calling out.
Within a regular expression, (?C) indicates the points at which the
external function is to be called. If you want to identify different
callout points, you can put a number less than 256 after the letter C.
The default value is zero. For example, this pattern has two callout
points:
(?C1)abc(?C2)def
If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
automatically installed before each item in the pattern. They are all
numbered 255.
During matching, when PCRE reaches a callout point (and pcre_callout is
set), the external function is called. It is provided with the number
of the callout, the position in the pattern, and, optionally, one item
of data originally supplied by the caller of pcre_exec(). The callout
function may cause matching to proceed, to backtrack, or to fail alto-
gether. A complete description of the interface to the callout function
is given in the pcrecallout documentation.
BACKTRACKING CONTROL
Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
which are described in the Perl documentation as "experimental and sub-
ject to change or removal in a future version of Perl". It goes on to
say: "Their usage in production code should be noted to avoid problems
during upgrades." The same remarks apply to the PCRE features described
in this section.
Since these verbs are specifically related to backtracking, most of
them can be used only when the pattern is to be matched using
pcre_exec(), which uses a backtracking algorithm. With the exception of
(*FAIL), which behaves like a failing negative assertion, they cause an
error if encountered by pcre_dfa_exec().
The new verbs make use of what was previously invalid syntax: an open-
ing parenthesis followed by an asterisk. In Perl, they are generally of
the form (*VERB:ARG) but PCRE does not support the use of arguments, so
its general form is just (*VERB). Any number of these verbs may occur
in a pattern. There are two kinds:
Verbs that act immediately
The following verbs act as soon as they are encountered:
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder
of the pattern. When inside a recursion, only the innermost pattern is
ended immediately. PCRE differs from Perl in what happens if the
(*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
captured: in PCRE no data is captured. For example:
A(A|B(*ACCEPT)|C)D
This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
is captured.
(*FAIL) or (*F)
This verb causes the match to fail, forcing backtracking to occur. It
is equivalent to (?!) but easier to read. The Perl documentation notes
that it is probably useful only when combined with (?{}) or (??{}).
Those are, of course, Perl features that are not present in PCRE. The
nearest equivalent is the callout feature, as for example in this pat-
tern:
a+(?C)(*FAIL)
A match with the string "aaaa" always fails, but the callout is taken
before each backtrack happens (in this example, 10 times).
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con-
tinues with what follows, but if there is no subsequent match, a fail-
ure is forced. The verbs differ in exactly what kind of failure
occurs.
(*COMMIT)
This verb causes the whole match to fail outright if the rest of the
pattern does not match. Even if the pattern is unanchored, no further
attempts to find a match by advancing the start point take place. Once
(*COMMIT) has been passed, pcre_exec() is committed to finding a match
at the current starting point, or not at all. For example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a kind
of dynamic anchor, or "I've started, so I must finish."
(*PRUNE)
This verb causes the match to fail at the current position if the rest
of the pattern does not match. If the pattern is unanchored, the normal
"bumpalong" advance to the next starting character then happens. Back-
tracking can occur as usual to the left of (*PRUNE), or when matching
to the right of (*PRUNE), but if there is no match to the right, back-
tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
is just an alternative to an atomic group or possessive quantifier, but
there are some uses of (*PRUNE) that cannot be expressed in any other
way.
(*SKIP)
This verb is like (*PRUNE), except that if the pattern is unanchored,
the "bumpalong" advance is not to the next character, but to the posi-
tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
that whatever text was matched leading up to it cannot be part of a
successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting point
skips on to start the next attempt at "c". Note that a possessive quan-
tifer does not have the same effect in this example; although it would
suppress backtracking during the first match attempt, the second
attempt would start at the second character instead of skipping on to
"c".
(*THEN)
This verb causes a skip to the next alternation if the rest of the pat-
tern does not match. That is, it cancels pending backtracking, but only
within the current alternation. Its name comes from the observation
that it can be used for a pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items
after the end of the group if FOO succeeds); on failure the matcher
skips to the second alternative and tries COND2, without backtracking
into COND1. If (*THEN) is used outside of any alternation, it acts
exactly like (*PRUNE).
SEE ALSO
pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 19 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRESYNTAX(3) PCRESYNTAX(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE REGULAR EXPRESSION SYNTAX SUMMARY
The full syntax and semantics of the regular expressions that are sup-
ported by PCRE are described in the pcrepattern documentation. This
document contains just a quick-reference summary of the syntax.
QUOTING
\x where x is non-alphanumeric is a literal x
\Q...\E treat enclosed characters as literal
CHARACTERS
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\ddd character with octal code ddd, or backreference
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
CHARACTER TYPES
. any character except newline;
in dotall mode, any character whatsoever
\C one byte, even in UTF-8 mode (best avoided)
\d a decimal digit
\D a character that is not a decimal digit
\h a horizontal whitespace character
\H a character that is not a horizontal whitespace character
\p{xx} a character with the xx property
\P{xx} a character without the xx property
\R a newline sequence
\s a whitespace character
\S a character that is not a whitespace character
\v a vertical whitespace character
\V a character that is not a vertical whitespace character
\w a "word" character
\W a "non-word" character
\X an extended Unicode sequence
In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
GENERAL CATEGORY PROPERTY CODES FOR \p and \P
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
L& Ll, Lu, or Lt
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
SCRIPT NAMES FOR \p AND \P
Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
CHARACTER CLASSES
[...] positive character class
[^...] negative character class
[x-y] range (can be used for hex characters)
[[:xxx:]] positive POSIX named set
[[:^xxx:]] negative POSIX named set
alnum alphanumeric
alpha alphabetic
ascii 0-127
blank space or tab
cntrl control character
digit decimal digit
graph printing, excluding space
lower lower case letter
print printing, including space
punct printing, excluding alphanumeric
space whitespace
upper upper case letter
word same as \w
xdigit hexadecimal digit
In PCRE, POSIX character set names recognize only ASCII characters. You
can use \Q...\E inside a character class.
QUANTIFIERS
? 0 or 1, greedy
?+ 0 or 1, possessive
?? 0 or 1, lazy
* 0 or more, greedy
*+ 0 or more, possessive
*? 0 or more, lazy
+ 1 or more, greedy
++ 1 or more, possessive
+? 1 or more, lazy
{n} exactly n
{n,m} at least n, no more than m, greedy
{n,m}+ at least n, no more than m, possessive
{n,m}? at least n, no more than m, lazy
{n,} n or more, greedy
{n,}+ n or more, possessive
{n,}? n or more, lazy
ANCHORS AND SIMPLE ASSERTIONS
\b word boundary
\B not a word boundary
^ start of subject
also after internal newline in multiline mode
\A start of subject
$ end of subject
also before newline at end of subject
also before internal newline in multiline mode
\Z end of subject
also before newline at end of subject
\z end of subject
\G first matching position in subject
MATCH POINT RESET
\K reset start of match
ALTERNATION
expr|expr|expr...
CAPTURING
(...) capturing group
(?<name>...) named capturing group (Perl)
(?'name'...) named capturing group (Perl)
(?P<name>...) named capturing group (Python)
(?:...) non-capturing group
(?|...) non-capturing group; reset group numbers for
capturing groups in each alternative
ATOMIC GROUPS
(?>...) atomic, non-capturing group
COMMENT
(?#....) comment (not nestable)
OPTION SETTING
(?i) caseless
(?J) allow duplicate names
(?m) multiline
(?s) single line (dotall)
(?U) default ungreedy (lazy)
(?x) extended (ignore white space)
(?-...) unset option(s)
LOOKAHEAD AND LOOKBEHIND ASSERTIONS
(?=...) positive look ahead
(?!...) negative look ahead
(?<=...) positive look behind
(?<!...) negative look behind
Each top-level branch of a look behind must be of a fixed length.
BACKREFERENCES
\n reference by number (can be ambiguous)
\gn reference by number
\g{n} reference by number
\g{-n} relative reference by number
\k<name> reference by name (Perl)
\k'name' reference by name (Perl)
\g{name} reference by name (Perl)
\k{name} reference by name (.NET)
(?P=name) reference by name (Python)
SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
(?R) recurse whole pattern
(?n) call subpattern by absolute number
(?+n) call subpattern by relative number
(?-n) call subpattern by relative number
(?&name;) call subpattern by name (Perl)
(?P>name) call subpattern by name (Python)
\g<name> call subpattern by name (Oniguruma)
\g'name' call subpattern by name (Oniguruma)
\g<n> call subpattern by absolute number (Oniguruma)
\g'n' call subpattern by absolute number (Oniguruma)
\g<+n> call subpattern by relative number (PCRE extension)
\g'+n' call subpattern by relative number (PCRE extension)
\g<-n> call subpattern by relative number (PCRE extension)
\g'-n' call subpattern by relative number (PCRE extension)
CONDITIONAL PATTERNS
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
(?(n)... absolute reference condition
(?(+n)... relative reference condition
(?(-n)... relative reference condition
(?(<name>)... named reference condition (Perl)
(?('name')... named reference condition (Perl)
(?(name)... named reference condition (PCRE)
(?(R)... overall recursion condition
(?(Rn)... specific group recursion condition
(?(R&name;)... specific recursion condition
(?(DEFINE)... define subpattern for reference
(?(assert)... assertion condition
BACKTRACKING CONTROL
The following act immediately they are reached:
(*ACCEPT) force successful match
(*FAIL) force backtrack; synonym (*F)
The following act only when a subsequent match failure causes a back-
track to reach them. They all force a match failure, but they differ in
what happens afterwards. Those that advance the start-of-match point do
so only if the pattern is not anchored.
(*COMMIT) overall failure, no advance of starting point
(*PRUNE) advance to next starting character
(*SKIP) advance start to current matching position
(*THEN) local failure, backtrack to next alternation
NEWLINE CONVENTIONS
These are recognized only at the very start of the pattern or after a
(*BSR_...) option.
(*CR)
(*LF)
(*CRLF)
(*ANYCRLF)
(*ANY)
WHAT \R MATCHES
These are recognized only at the very start of the pattern or after a
(*...) option that sets the newline convention.
(*BSR_ANYCRLF)
(*BSR_UNICODE)
CALLOUTS
(?C) callout
(?Cn) callout with data n
SEE ALSO
pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 09 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCREPARTIAL(3) PCREPARTIAL(3)
NAME
PCRE - Perl-compatible regular expressions
PARTIAL MATCHING IN PCRE
In normal use of PCRE, if the subject string that is passed to
pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
There are circumstances where it might be helpful to distinguish this
case from other cases in which there is no match.
Consider, for example, an application where a human is required to type
in data for a field with specific formatting requirements. An example
might be a date in the form ddmmmyy, defined by this pattern:
^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
If the application sees the user's keystrokes one by one, and can check
that what has been typed so far is potentially valid, it is able to
raise an error as soon as a mistake is made, possibly beeping and not
reflecting the character that has been typed. This immediate feedback
is likely to be a better user interface than a check that is delayed
until the entire string has been entered.
PCRE supports the concept of partial matching by means of the PCRE_PAR-
TIAL option, which can be set when calling pcre_exec() or
pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
during the matching process the last part of the subject string matched
part of the pattern. Unfortunately, for non-anchored matching, it is
not possible to obtain the position of the start of the partial match.
No captured data is set when PCRE_ERROR_PARTIAL is returned.
When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
the subject is reached, there have been no complete matches, but there
is still at least one matching possibility. The portion of the string
that provided the partial match is set as the first matching string.
Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
the last literal byte in a pattern, and abandons matching immediately
if such a byte is not present in the subject string. This optimization
cannot be used for a subject string that might match only partially.
RESTRICTED PATTERNS FOR PCRE_PARTIAL
Because of the way certain internal optimizations are implemented in
the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
all patterns. These restrictions do not apply when pcre_dfa_exec() is
used. For pcre_exec(), repeated single characters such as
a{2,4}
and repeated single metasequences such as
\d+
are not permitted if the maximum number of occurrences is greater than
one. Optional items such as \d? (where the maximum is one) are permit-
ted. Quantifiers with any values are permitted after parentheses, so
the invalid examples above can be coded thus:
(a){2,4}
(\d)+
These constructions run more slowly, but for the kinds of application
that are envisaged for this facility, this is not felt to be a major
restriction.
If PCRE_PARTIAL is set for a pattern that does not conform to the
restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
(-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
find out if a compiled pattern can be used for partial matching.
EXAMPLE OF PARTIAL MATCHING USING PCRETEST
If the escape sequence \P is present in a pcretest data line, the
PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
uses the date example quoted above:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\P
0: 25jun04
1: jun
data> 25dec3\P
Partial match
data> 3ju\P
Partial match
data> 3juj\P
No match
data> j\P
No match
The first data string is matched completely, so pcretest shows the
matched substrings. The remaining four strings do not match the com-
plete pattern, but the first two are partial matches. The same test,
using pcre_dfa_exec() matching (by means of the \D escape sequence),
produces the following output:
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 25jun04\P\D
0: 25jun04
data> 23dec3\P\D
Partial match: 23dec3
data> 3ju\P\D
Partial match: 3ju
data> 3juj\P\D
No match
data> j\P\D
No match
Notice that in this case the portion of the string that was matched is
made available.
MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
When a partial match has been found using pcre_dfa_exec(), it is possi-
ble to continue the match by providing additional subject data and
calling pcre_dfa_exec() again with the same compiled regular expres-
sion, this time setting the PCRE_DFA_RESTART option. You must also pass
the same working space as before, because this is where details of the
previous partial match are stored. Here is an example using pcretest,
using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
\D are as above):
re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
data> 23ja\P\D
Partial match: 23ja
data> n05\R\D
0: n05
The first call has "23ja" as the subject, and requests partial match-
ing; the second call has "n05" as the subject for the continued
(restarted) match. Notice that when the match is complete, only the
last part is shown; PCRE does not retain the previously partially-
matched string. It is up to the calling program to do that if it needs
to.
You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
matching over multiple segments. This facility can be used to pass very
long subject strings to pcre_dfa_exec(). However, some care is needed
for certain types of pattern.
1. If the pattern contains tests for the beginning or end of a line,
you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
ate, when the subject string for any call does not contain the begin-
ning or end of a line.
2. If the pattern contains backward assertions (including \b or \B),
you need to arrange for some overlap in the subject strings to allow
for this. For example, you could pass the subject in chunks that are
500 bytes long, but in a buffer of 700 bytes, with the starting offset
set to 200 and the previous 200 bytes at the start of the buffer.
3. Matching a subject string that is split into multiple segments does
not always produce exactly the same result as matching over one single
long string. The difference arises when there are multiple matching
possibilities, because a partial match result is given only when there
are no completed matches in a call to pcre_dfa_exec(). This means that
as soon as the shortest match has been found, continuation to a new
subject segment is no longer possible. Consider this pcretest example:
re> /dog(sbody)?/
data> do\P\D
Partial match: do
data> gsb\R\P\D
0: g
data> dogsbody\D
0: dogsbody
1: dog
The pattern matches the words "dog" or "dogsbody". When the subject is
presented in several parts ("do" and "gsb" being the first two) the
match stops when "dog" has been found, and it is not possible to con-
tinue. On the other hand, if "dogsbody" is presented as a single
string, both matches are found.
Because of this phenomenon, it does not usually make sense to end a
pattern that is going to be matched in this way with a variable repeat.
4. Patterns that contain alternatives at the top level which do not all
start with the same pattern item may not work as expected. For example,
consider this pattern:
1234|3789
If the first part of the subject is "ABC123", a partial match of the
first alternative is found at offset 3. There is no partial match for
the second alternative, because such a match does not start at the same
point in the subject string. Attempting to continue with the string
"789" does not yield a match because only those alternatives that match
at one point in the subject are remembered. The problem arises because
the start of the second alternative matches within the first alterna-
tive. There is no problem with anchored patterns or patterns such as:
1234|ABCD
where no string can be a partial match for both alternatives.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 04 June 2007
Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------
PCREPRECOMPILE(3) PCREPRECOMPILE(3)
NAME
PCRE - Perl-compatible regular expressions
SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
If you are running an application that uses a large number of regular
expression patterns, it may be useful to store them in a precompiled
form instead of having to compile them every time the application is
run. If you are not using any private character tables (see the
pcre_maketables() documentation), this is relatively straightforward.
If you are using private tables, it is a little bit more complicated.
If you save compiled patterns to a file, you can copy them to a differ-
ent host and run them there. This works even if the new host has the
opposite endianness to the one on which the patterns were compiled.
There may be a small performance penalty, but it should be insignifi-
cant. However, compiling regular expressions with one version of PCRE
for use with a different version is not guaranteed to work and may
cause crashes.
SAVING A COMPILED PATTERN
The value returned by pcre_compile() points to a single block of memory
that holds the compiled pattern and associated data. You can find the
length of this block in bytes by calling pcre_fullinfo() with an argu-
ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
manner. Here is sample code that compiles a pattern and writes it to a
file. It assumes that the variable fd refers to a file that is open for
output:
int erroroffset, rc, size;
char *error;
pcre *re;
re = pcre_compile("my pattern", 0, &error;, &erroroffset;, NULL);
if (re == NULL) { ... handle errors ... }
rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size;);
if (rc < 0) { ... handle errors ... }
rc = fwrite(re, 1, size, fd);
if (rc != size) { ... handle errors ... }
In this example, the bytes that comprise the compiled pattern are
copied exactly. Note that this is binary data that may contain any of
the 256 possible byte values. On systems that make a distinction
between binary and non-binary data, be sure that the file is opened for
binary output.
If you want to write more than one pattern to a file, you will have to
devise a way of separating them. For binary data, preceding each pat-
tern with its length is probably the most straightforward approach.
Another possibility is to write out the data in hexadecimal instead of
binary, one pattern to a line.
Saving compiled patterns in a file is only one possible way of storing
them for later use. They could equally well be saved in a database, or
in the memory of some daemon process that passes them via sockets to
the processes that want them.
If the pattern has been studied, it is also possible to save the study
data in a similar way to the compiled pattern itself. When studying
generates additional information, pcre_study() returns a pointer to a
pcre_extra data block. Its format is defined in the section on matching
a pattern in the pcreapi documentation. The study_data field points to
the binary study data, and this is what you must save (not the
pcre_extra block itself). The length of the study data can be obtained
by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
Remember to check that pcre_study() did return a non-NULL value before
trying to save the study data.
RE-USING A PRECOMPILED PATTERN
Re-using a precompiled pattern is straightforward. Having reloaded it
into main memory, you pass its pointer to pcre_exec() or
pcre_dfa_exec() in the usual way. This should work even on another
host, and even if that host has the opposite endianness to the one
where the pattern was compiled.
However, if you passed a pointer to custom character tables when the
pattern was compiled (the tableptr argument of pcre_compile()), you
must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
because the value saved with the compiled pattern will obviously be
nonsense. A field in a pcre_extra() block is used to pass this data, as
described in the section on matching a pattern in the pcreapi documen-
tation.
If you did not provide custom character tables when the pattern was
compiled, the pointer in the compiled pattern is NULL, which causes
pcre_exec() to use PCRE's internal tables. Thus, you do not need to
take any special action at run time in this case.
If you saved study data with the compiled pattern, you need to create
your own pcre_extra data block and set the study_data field to point to
the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
bit in the flags field to indicate that study data is present. Then
pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
usual way.
COMPATIBILITY WITH DIFFERENT PCRE RELEASES
In general, it is safest to recompile all saved patterns when you
update to a new PCRE release, though not all updates actually require
this. Recompiling is definitely needed for release 7.2.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 13 June 2007
Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------
PCREPERFORM(3) PCREPERFORM(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE PERFORMANCE
Two aspects of performance are discussed below: memory usage and pro-
cessing time. The way you express your pattern as a regular expression
can affect both of them.
MEMORY USAGE
Patterns are compiled by PCRE into a reasonably efficient byte code, so
that most simple patterns do not use much memory. However, there is one
case where memory usage can be unexpectedly large. When a parenthesized
subpattern has a quantifier with a minimum greater than 1 and/or a lim-
ited maximum, the whole subpattern is repeated in the compiled code.
For example, the pattern
(abc|def){2,4}
is compiled as if it were
(abc|def)(abc|def)((abc|def)(abc|def)?)?
(Technical aside: It is done this way so that backtrack points within
each of the repetitions can be independently maintained.)
For regular expressions whose quantifiers use only small numbers, this
is not usually a problem. However, if the numbers are large, and par-
ticularly if such repetitions are nested, the memory usage can become
an embarrassment. For example, the very simple pattern
((ab){1,1000}c){1,3}
uses 51K bytes when compiled. When PCRE is compiled with its default
internal pointer size of two bytes, the size limit on a compiled pat-
tern is 64K, and this is reached with the above pattern if the outer
repetition is increased from 3 to 4. PCRE can be compiled to use larger
internal pointers and thus handle larger compiled patterns, but it is
better to try to rewrite your pattern to use less memory if you can.
One way of reducing the memory usage for such patterns is to make use
of PCRE's "subroutine" facility. Re-writing the above pattern as
((ab)(?2){0,999}c)(?1){0,2}
reduces the memory requirements to 18K, and indeed it remains under 20K
even with the outer repetition increased to 100. However, this pattern
is not exactly equivalent, because the "subroutine" calls are treated
as atomic groups into which there can be no backtracking if there is a
subsequent matching failure. Therefore, PCRE cannot do this kind of
rewriting automatically. Furthermore, there is a noticeable loss of
speed when executing the modified pattern. Nevertheless, if the atomic
grouping is not a problem and the loss of speed is acceptable, this
kind of rewriting will allow you to process patterns that PCRE cannot
otherwise handle.
PROCESSING TIME
Certain items in regular expression patterns are processed more effi-
ciently than others. It is more efficient to use a character class like
[aeiou] than a set of single-character alternatives such as
(a|e|i|o|u). In general, the simplest construction that provides the
required behaviour is usually the most efficient. Jeffrey Friedl's book
contains a lot of useful general discussion about optimizing regular
expressions for efficient performance. This document contains a few
observations about PCRE.
Using Unicode character properties (the \p, \P, and \X escapes) is
slow, because PCRE has to scan a structure that contains data for over
fifteen thousand characters whenever it needs a character's property.
If you can find an alternative pattern that does not use character
properties, it will probably be faster.
When a pattern begins with .* not in parentheses, or in parentheses
that are not the subject of a backreference, and the PCRE_DOTALL option
is set, the pattern is implicitly anchored by PCRE, since it can match
only at the start of a subject string. However, if PCRE_DOTALL is not
set, PCRE cannot make this optimization, because the . metacharacter
does not then match a newline, and if the subject string contains new-
lines, the pattern may match from the character immediately following
one of them instead of from the very start. For example, the pattern
.*second
matches the subject "first\nand second" (where \n stands for a newline
character), with the match starting at the seventh character. In order
to do this, PCRE has to retry the match starting after every newline in
the subject.
If you are using such a pattern with subject strings that do not con-
tain newlines, the best performance is obtained by setting PCRE_DOTALL,
or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
ing. That saves PCRE from having to scan along the subject looking for
a newline to restart at.
Beware of patterns that contain nested indefinite repeats. These can
take a long time to run when applied to a string that does not match.
Consider the pattern fragment
^(a+)*
This can match "aaaa" in 16 different ways, and this number increases
very rapidly as the string gets longer. (The * repeat can match 0, 1,
2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
repeats can match different numbers of times.) When the remainder of
the pattern is such that the entire match is going to fail, PCRE has in
principle to try every possible variation, and this can take an
extremely long time, even for relatively short strings.
An optimization catches some of the more simple cases such as
(a+)*b
where a literal character follows. Before embarking on the standard
matching procedure, PCRE checks that there is a "b" later in the sub-
ject string, and if there is not, it fails the match immediately. How-
ever, when there is no following literal this optimization cannot be
used. You can see the difference by comparing the behaviour of
(a+)*\d
with the pattern above. The former gives a failure almost instantly
when applied to a whole line of "a" characters, whereas the latter
takes an appreciable time with strings longer than about 20 characters.
In many cases, the solution to this kind of performance issue is to use
an atomic group or a possessive quantifier.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 06 March 2007
Copyright (c) 1997-2007 University of Cambridge.
------------------------------------------------------------------------------
PCREPOSIX(3) PCREPOSIX(3)
NAME
PCRE - Perl-compatible regular expressions.
SYNOPSIS OF POSIX API
#include <pcreposix.h>
int regcomp(regex_t *preg, const char *pattern,
int cflags);
int regexec(regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void regfree(regex_t *preg);
DESCRIPTION
This set of functions provides a POSIX-style API to the PCRE regular
expression package. See the pcreapi documentation for a description of
PCRE's native API, which contains much additional functionality.
The functions described here are just wrapper functions that ultimately
call the PCRE native API. Their prototypes are defined in the
pcreposix.h header file, and on Unix systems the library itself is
called pcreposix.a, so can be accessed by adding -lpcreposix to the
command for linking an application that uses them. Because the POSIX
functions call the native ones, it is also necessary to add -lpcre.
I have implemented only those option bits that can be reasonably mapped
to PCRE native options. In addition, the option REG_EXTENDED is defined
with the value zero. This has no effect, but since programs that are
written to the POSIX interface often use it, this makes it easier to
slot in PCRE as a replacement library. Other POSIX options are not even
defined.
When PCRE is called via these functions, it is only the API that is
POSIX-like in style. The syntax and semantics of the regular expres-
sions themselves are still those of Perl, subject to the setting of
various PCRE options, as described below. "POSIX-like in style" means
that the API approximates to the POSIX definition; it is not fully
POSIX-compatible, and in multi-byte encoding domains it is probably
even less compatible.
The header for these functions is supplied as pcreposix.h to avoid any
potential clash with other POSIX libraries. It can, of course, be
renamed or aliased as regex.h, which is the "correct" name. It provides
two structure types, regex_t for compiled internal forms, and reg-
match_t for returning captured substrings. It also defines some con-
stants whose names start with "REG_"; these are used for setting
options and identifying error codes.
COMPILING A PATTERN
The function regcomp() is called to compile a pattern into an internal
form. The pattern is a C string terminated by a binary zero, and is
passed in the argument pattern. The preg argument is a pointer to a
regex_t structure that is used as a base for storing information about
the compiled regular expression.
The argument cflags is either zero, or contains one or more of the bits
defined by the following macros:
REG_DOTALL
The PCRE_DOTALL option is set when the regular expression is passed for
compilation to the native function. Note that REG_DOTALL is not part of
the POSIX standard.
REG_ICASE
The PCRE_CASELESS option is set when the regular expression is passed
for compilation to the native function.
REG_NEWLINE
The PCRE_MULTILINE option is set when the regular expression is passed
for compilation to the native function. Note that this does not mimic
the defined POSIX behaviour for REG_NEWLINE (see the following sec-
tion).
REG_NOSUB
The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
passed for compilation to the native function. In addition, when a pat-
tern that is compiled with this flag is passed to regexec() for match-
ing, the nmatch and pmatch arguments are ignored, and no captured
strings are returned.
REG_UTF8
The PCRE_UTF8 option is set when the regular expression is passed for
compilation to the native function. This causes the pattern itself and
all data strings used for matching it to be treated as UTF-8 strings.
Note that REG_UTF8 is not part of the POSIX standard.
In the absence of these flags, no options are passed to the native
function. This means the the regex is compiled with PCRE default
semantics. In particular, the way it handles newline characters in the
subject string is the Perl way, not the POSIX way. Note that setting
PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
It does not affect the way newlines are matched by . (they aren't) or
by a negative class such as [^a] (they are).
The yield of regcomp() is zero on success, and non-zero otherwise. The
preg structure is filled in on success, and one member of the structure
is public: re_nsub contains the number of capturing subpatterns in the
regular expression. Various error codes are defined in the header file.
MATCHING NEWLINE CHARACTERS
This area is not simple, because POSIX and Perl take different views of
things. It is not possible to get PCRE to obey POSIX semantics, but
then PCRE was never intended to be a POSIX engine. The following table
lists the different possibilities for matching newline characters in
PCRE:
Default Change with
. matches newline no PCRE_DOTALL
newline matches [^a] yes not changeable
$ matches \n at end yes PCRE_DOLLARENDONLY
$ matches \n in middle no PCRE_MULTILINE
^ matches \n in middle no PCRE_MULTILINE
This is the equivalent table for POSIX:
Default Change with
. matches newline yes REG_NEWLINE
newline matches [^a] yes REG_NEWLINE
$ matches \n at end no REG_NEWLINE
$ matches \n in middle no REG_NEWLINE
^ matches \n in middle no REG_NEWLINE
PCRE's behaviour is the same as Perl's, except that there is no equiva-
lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
no way to stop newline from matching [^a].
The default POSIX newline handling can be obtained by setting
PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
behave exactly as for the REG_NEWLINE action.
MATCHING A PATTERN
The function regexec() is called to match a compiled pattern preg
against a given string, which is by default terminated by a zero byte
(but see REG_STARTEND below), subject to the options in eflags. These
can be:
REG_NOTBOL
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
function.
REG_NOTEOL
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
function.
REG_STARTEND
The string is considered to start at string + pmatch[0].rm_so and to
have a terminating NUL located at string + pmatch[0].rm_eo (there need
not actually be a NUL at that location), regardless of the value of
nmatch. This is a BSD extension, compatible with but not specified by
IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
software intended to be portable to other systems. Note that a non-zero
rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
of the string, not how it is matched.
If the pattern was compiled with the REG_NOSUB flag, no data about any
matched strings is returned. The nmatch and pmatch arguments of
regexec() are ignored.
Otherwise,the portion of the string that was matched, and also any cap-
tured substrings, are returned via the pmatch argument, which points to
an array of nmatch structures of type regmatch_t, containing the mem-
bers rm_so and rm_eo. These contain the offset to the first character
of each substring and the offset to the first character after the end
of each substring, respectively. The 0th element of the vector relates
to the entire portion of string that was matched; subsequent elements
relate to the capturing subpatterns of the regular expression. Unused
entries in the array have both structure members set to -1.
A successful match yields a zero return; various error codes are
defined in the header file, of which REG_NOMATCH is the "expected"
failure code.
ERROR MESSAGES
The regerror() function maps a non-zero errorcode from either regcomp()
or regexec() to a printable message. If preg is not NULL, the error
should have arisen from the use of that structure. A message terminated
by a binary zero is placed in errbuf. The length of the message,
including the zero, is limited to errbuf_size. The yield of the func-
tion is the size of buffer needed to hold the whole message.
MEMORY USAGE
Compiling a regular expression causes memory to be allocated and asso-
ciated with the preg structure. The function regfree() frees all such
memory, after which preg may no longer be used as a compiled expres-
sion.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 05 April 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRECPP(3) PCRECPP(3)
NAME
PCRE - Perl-compatible regular expressions.
SYNOPSIS OF C++ WRAPPER
#include <pcrecpp.h>
DESCRIPTION
The C++ wrapper for PCRE was provided by Google Inc. Some additional
functionality was added by Giuseppe Maxia. This brief man page was con-
structed from the notes in the pcrecpp.h file, which should be con-
sulted for further details.
MATCHING INTERFACE
The "FullMatch" operation checks that supplied text matches a supplied
pattern exactly. If pointer arguments are supplied, it copies matched
sub-strings that match sub-patterns into them.
Example: successful match
pcrecpp::RE re("h.*o");
re.FullMatch("hello");
Example: unsuccessful match (requires full match):
pcrecpp::RE re("e");
!re.FullMatch("hello");
Example: creating a temporary RE object:
pcrecpp::RE("h.*o").FullMatch("hello");
You can pass in a "const char*" or a "string" for "text". The examples
below tend to use a const char*. You can, as in the different examples
above, store the RE object explicitly in a variable or use a temporary
RE object. The examples below use one mode or the other arbitrarily.
Either could correctly be used for any of these examples.
You must supply extra pointer arguments to extract matched subpieces.
Example: extracts "ruby" into "s" and 1234 into "i"
int i;
string s;
pcrecpp::RE re("(\\w+):(\\d+)");
re.FullMatch("ruby:1234", &s;, &i;);
Example: does not try to extract any extra sub-patterns
re.FullMatch("ruby:1234", &s;);
Example: does not try to extract into NULL
re.FullMatch("ruby:1234", NULL, &i;);
Example: integer overflow causes failure
!re.FullMatch("ruby:1234567891234", NULL, &i;);
Example: fails because there aren't enough sub-patterns:
!pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s;);
Example: fails because string cannot be stored in integer
!pcrecpp::RE("(.*)").FullMatch("ruby", &i;);
The provided pointer arguments can be pointers to any scalar numeric
type, or one of:
string (matched piece is copied to string)
StringPiece (StringPiece is mutated to point to matched piece)
T (where "bool T::ParseFrom(const char*, int)" exists)
NULL (the corresponding matched sub-pattern is not copied)
The function returns true iff all of the following conditions are sat-
isfied:
a. "text" matches "pattern" exactly;
b. The number of matched sub-patterns is >= number of supplied
pointers;
c. The "i"th argument has a suitable type for holding the
string captured as the "i"th sub-pattern. If you pass in
void * NULL for the "i"th argument, or a non-void * NULL
of the correct type, or pass fewer arguments than the
number of sub-patterns, "i"th captured sub-pattern is
ignored.
CAVEAT: An optional sub-pattern that does not exist in the matched
string is assigned the empty string. Therefore, the following will
return false (because the empty string is not a valid number):
int number;
pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number;);
The matching interface supports at most 16 arguments per call. If you
need more, consider using the more general interface
pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
QUOTING METACHARACTERS
You can use the "QuoteMeta" operation to insert backslashes before all
potentially meaningful characters in a string. The returned string,
used as a regular expression, will exactly match the original string.
Example:
string quoted = RE::QuoteMeta(unquoted);
Note that it's legal to escape a character even if it has no special
meaning in a regular expression -- so this function does that. (This
also makes it identical to the perl function of the same name; see
"perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
"1\.5\-2\.0\?".
PARTIAL MATCHES
You can use the "PartialMatch" operation when you want the pattern to
match any substring of the text.
Example: simple search for a string:
pcrecpp::RE("ell").PartialMatch("hello");
Example: find first number in a string:
int number;
pcrecpp::RE re("(\\d+)");
re.PartialMatch("x*100 + 20", &number;);
assert(number == 100);
UTF-8 AND THE MATCHING INTERFACE
By default, pattern and text are plain text, one byte per character.
The UTF8 flag, passed to the constructor, causes both pattern and
string to be treated as UTF-8 text, still a byte stream but potentially
multiple bytes per character. In practice, the text is likelier to be
UTF-8 than the pattern, but the match returned may depend on the UTF8
flag, so always use it when matching UTF8 text. For example, "." will
match one byte normally but with UTF8 set may match up to three bytes
of a multi-byte character.
Example:
pcrecpp::RE_Options options;
options.set_utf8();
pcrecpp::RE re(utf8_pattern, options);
re.FullMatch(utf8_string);
Example: using the convenience function UTF8():
pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
re.FullMatch(utf8_string);
NOTE: The UTF8 flag is ignored if pcre was not configured with the
--enable-utf8 flag.
PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
PCRE defines some modifiers to change the behavior of the regular
expression engine. The C++ wrapper defines an auxiliary class,
RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
rently, the following modifiers are supported:
modifier description Perl corresponding
PCRE_CASELESS case insensitive match /i
PCRE_MULTILINE multiple lines match /m
PCRE_DOTALL dot matches newlines /s
PCRE_DOLLAR_ENDONLY $ matches only at end N/A
PCRE_EXTRA strict escape parsing N/A
PCRE_EXTENDED ignore whitespaces /x
PCRE_UTF8 handles UTF8 chars built-in
PCRE_UNGREEDY reverses * and *? N/A
PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
(*) Both Perl and PCRE allow non capturing parentheses by means of the
"?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
ture, while (ab|cd) does.
For a full account on how each modifier works, please check the PCRE
API reference page.
For each modifier, there are two member functions whose name is made
out of the modifier in lowercase, without the "PCRE_" prefix. For
instance, PCRE_CASELESS is handled by
bool caseless()
which returns true if the modifier is set, and
RE_Options & set_caseless(bool)
which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
be accessed through the set_match_limit() and match_limit() member
functions. Setting match_limit to a non-zero value will limit the exe-
cution of pcre to keep it from doing bad things like blowing the stack
or taking an eternity to return a result. A value of 5000 is good
enough to stop stack blowup in a 2MB thread stack. Setting match_limit
to zero disables match limiting. Alternatively, you can call
match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
limit how much PCRE recurses. match_limit() limits the number of
matches PCRE does; match_limit_recursion() limits the depth of internal
recursion, and therefore the amount of stack that is used.
Normally, to pass one or more modifiers to a RE class, you declare a
RE_Options object, set the appropriate options, and pass this object to
a RE constructor. Example:
RE_options opt;
opt.set_caseless(true);
if (RE("HELLO", opt).PartialMatch("hello world")) ...
RE_options has two constructors. The default constructor takes no argu-
ments and creates a set of flags that are off by default. The optional
parameter option_flags is to facilitate transfer of legacy code from C
programs. This lets you do
RE(pattern,
RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
However, new code is better off doing
RE(pattern,
RE_Options().set_caseless(true).set_multiline(true))
.PartialMatch(str);
If you are going to pass one of the most used modifiers, there are some
convenience functions that return a RE_Options class with the appropri-
ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
and EXTENDED().
If you need to set several options at once, and you don't want to go
through the pains of declaring a RE_Options object and setting several
options, there is a parallel method that give you such ability on the
fly. You can concatenate several set_xxxxx() member functions, since
each of them returns a reference to its class object. For example, to
pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
statement, you may write:
RE(" ^ xyz \\s+ .* blah$",
RE_Options()
.set_caseless(true)
.set_extended(true)
.set_multiline(true)).PartialMatch(sometext);
SCANNING TEXT INCREMENTALLY
The "Consume" operation may be useful if you want to repeatedly match
regular expressions at the front of a string and skip over them as they
match. This requires use of the "StringPiece" type, which represents a
sub-range of a real string. Like RE, StringPiece is defined in the
pcrecpp namespace.
Example: read lines of the form "var = value" from a string.
string contents = ...; // Fill string somehow
pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
string var;
int value;
pcrecpp::RE re("(\\w+) = (\\d+)\n");
while (re.Consume(&input;, &var;, &value;)) {
...;
}
Each successful call to "Consume" will set "var/value", and also
advance "input" so it points past the matched text.
The "FindAndConsume" operation is similar to "Consume" but does not
anchor your match at the beginning of the string. For example, you
could extract all words from a string by repeatedly calling
pcrecpp::RE("(\\w+)").FindAndConsume(&input;, &word;)
PARSING HEX/OCTAL/C-RADIX NUMBERS
By default, if you pass a pointer to a numeric value, the corresponding
text is interpreted as a base-10 number. You can instead wrap the
pointer with a call to one of the operators Hex(), Octal(), or CRadix()
to interpret the text in another base. The CRadix operator interprets
C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
base-10.
Example:
int a, b, c, d;
pcrecpp::RE re("(.*) (.*) (.*) (.*)");
re.FullMatch("100 40 0100 0x40",
pcrecpp::Octal(&a;), pcrecpp::Hex(&b;),
pcrecpp::CRadix(&c;), pcrecpp::CRadix(&d;));
will leave 64 in a, b, c, and d.
REPLACING PARTS OF STRINGS
You can replace the first match of "pattern" in "str" with "rewrite".
Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
insert text matching corresponding parenthesized group from the pat-
tern. \0 in "rewrite" refers to the entire matching text. For example:
string s = "yabba dabba doo";
pcrecpp::RE("b+").Replace("d", &s;);
will leave "s" containing "yada dabba doo". The result is true if the
pattern matches and a replacement occurs, false otherwise.
GlobalReplace is like Replace except that it replaces all occurrences
of the pattern in the string with the rewrite. Replacements are not
subject to re-matching. For example:
string s = "yabba dabba doo";
pcrecpp::RE("b+").GlobalReplace("d", &s;);
will leave "s" containing "yada dada doo". It returns the number of
replacements made.
Extract is like Replace, except that if the pattern matches, "rewrite"
is copied into "out" (an additional argument) with substitutions. The
non-matching portions of "text" are ignored. Returns true iff a match
occurred and the extraction happened successfully; if no match occurs,
the string is left unaffected.
AUTHOR
The C++ wrapper was contributed by Google Inc.
Copyright (c) 2007 Google Inc.
REVISION
Last updated: 12 November 2007
------------------------------------------------------------------------------
PCRESAMPLE(3) PCRESAMPLE(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE SAMPLE PROGRAM
A simple, complete demonstration program, to get you started with using
PCRE, is supplied in the file pcredemo.c in the PCRE distribution.
The program compiles the regular expression that is its first argument,
and matches it against the subject string in its second argument. No
PCRE options are set, and default character tables are used. If match-
ing succeeds, the program outputs the portion of the subject that
matched, together with the contents of any captured substrings.
If the -g option is given on the command line, the program then goes on
to check for further matches of the same regular expression in the same
subject string. The logic is a little bit tricky because of the possi-
bility of matching an empty string. Comments in the code explain what
is going on.
If PCRE is installed in the standard include and library directories
for your system, you should be able to compile the demonstration pro-
gram using this command:
gcc -o pcredemo pcredemo.c -lpcre
If PCRE is installed elsewhere, you may need to add additional options
to the command line. For example, on a Unix-like system that has PCRE
installed in /usr/local, you can compile the demonstration program
using a command like this:
gcc -o pcredemo -I/usr/local/include pcredemo.c \
-L/usr/local/lib -lpcre
Once you have compiled the demonstration program, you can run simple
tests like this:
./pcredemo 'cat|dog' 'the cat sat on the mat'
./pcredemo -g 'cat|dog' 'the dog sat on the cat'
Note that there is a much more comprehensive test program, called
pcretest, which supports many more facilities for testing regular
expressions and the PCRE library. The pcredemo program is provided as a
simple coding example.
On some operating systems (e.g. Solaris), when PCRE is not installed in
the standard library directory, you may get an error like this when you
try to run pcredemo:
ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
directory
This is caused by the way shared library support works on those sys-
tems. You need to add
-R/usr/local/lib
(for example) to the compile command to get round this problem.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 23 January 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------
PCRESTACK(3) PCRESTACK(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE DISCUSSION OF STACK USAGE
When you call pcre_exec(), it makes use of an internal function called
match(). This calls itself recursively at branch points in the pattern,
in order to remember the state of the match so that it can back up and
try a different alternative if the first one fails. As matching pro-
ceeds deeper and deeper into the tree of possibilities, the recursion
depth increases.
Not all calls of match() increase the recursion depth; for an item such
as a* it may be called several times at the same level, after matching
different numbers of a's. Furthermore, in a number of cases where the
result of the recursive call would immediately be passed back as the
result of the current call (a "tail recursion"), the function is just
restarted instead.
The pcre_dfa_exec() function operates in an entirely different way, and
hardly uses recursion at all. The limit on its complexity is the amount
of workspace it is given. The comments that follow do NOT apply to
pcre_dfa_exec(); they are relevant only for pcre_exec().
You can set limits on the number of times that match() is called, both
in total and recursively. If the limit is exceeded, an error occurs.
For details, see the section on extra data for pcre_exec() in the
pcreapi documentation.
Each time that match() is actually called recursively, it uses memory
from the process stack. For certain kinds of pattern and data, very
large amounts of stack may be needed, despite the recognition of "tail
recursion". You can often reduce the amount of recursion, and there-
fore the amount of stack used, by modifying the pattern that is being
matched. Consider, for example, this pattern:
([^<]|<(?!inet))+
It matches from wherever it starts until it encounters "<inet" or the
end of the data, and is the kind of pattern that might be used when
processing an XML file. Each iteration of the outer parentheses matches
either one character that is not "<" or a "<" that is not followed by
"inet". However, each time a parenthesis is processed, a recursion
occurs, so this formulation uses a stack frame for each matched charac-
ter. For a long string, a lot of stack is required. Consider now this
rewritten pattern, which matches exactly the same strings:
([^<]++|<(?!inet))+
This uses very much less stack, because runs of characters that do not
contain "<" are "swallowed" in one item inside the parentheses. Recur-
sion happens only when a "<" character that is not followed by "inet"
is encountered (and we assume this is relatively rare). A possessive
quantifier is used to stop any backtracking into the runs of non-"<"
characters, but that is not related to stack usage.
This example shows that one way of avoiding stack problems when match-
ing long subject strings is to write repeated parenthesized subpatterns
to match more than one character whenever possible.
Compiling PCRE to use heap instead of stack
In environments where stack memory is constrained, you might want to
compile PCRE to use heap memory instead of stack for remembering back-
up points. This makes it run a lot more slowly, however. Details of how
to do this are given in the pcrebuild documentation. When built in this
way, instead of using the stack, PCRE obtains and frees memory by call-
ing the functions that are pointed to by the pcre_stack_malloc and
pcre_stack_free variables. By default, these point to malloc() and
free(), but you can replace the pointers to cause PCRE to use your own
functions. Since the block sizes are always the same, and are always
freed in reverse order, it may be possible to implement customized mem-
ory handlers that are more efficient than the standard functions.
Limiting PCRE's stack usage
PCRE has an internal counter that can be used to limit the depth of
recursion, and thus cause pcre_exec() to give an error code before it
runs out of stack. By default, the limit is very large, and unlikely
ever to operate. It can be changed when PCRE is built, and it can also
be set when pcre_exec() is called. For details of these interfaces, see
the pcrebuild and pcreapi documentation.
As a very rough rule of thumb, you should reckon on about 500 bytes per
recursion. Thus, if you want to limit your stack usage to 8Mb, you
should set the limit at 16000 recursions. A 64Mb stack, on the other
hand, can support around 128000 recursions. The pcretest test program
has a command line option (-S) that can be used to increase the size of
its stack.
Changing stack size in Unix-like systems
In Unix-like environments, there is not often a problem with the stack
unless very long strings are involved, though the default limit on
stack size varies from system to system. Values from 8Mb to 64Mb are
common. You can find your default limit by running the command:
ulimit -s
Unfortunately, the effect of running out of stack is often SIGSEGV,
though sometimes a more explicit error message is given. You can nor-
mally increase the limit on stack size by code such as this:
struct rlimit rlim;
getrlimit(RLIMIT_STACK, &rlim;);
rlim.rlim_cur = 100*1024*1024;
setrlimit(RLIMIT_STACK, &rlim;);
This reads the current limits (soft and hard) using getrlimit(), then
attempts to increase the soft limit to 100Mb using setrlimit(). You
must do this before calling pcre_exec().
Changing stack size in Mac OS X
Using setrlimit(), as described above, should also work on Mac OS X. It
is also possible to set a stack size when linking a program. There is a
discussion about stack sizes in Mac OS X at this web site:
http://developer.apple.com/qa/qa2005/qa1419.html.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 09 July 2008
Copyright (c) 1997-2008 University of Cambridge.
------------------------------------------------------------------------------