Multiline. Views Haystack as a collection of individual lines (if it contains newlines) rather than as a single continuous line. Specifically, it changes the following:

1) Circumflex (^) matches immediately after all internal newlines -- as well as at the start of Haystack where it always matches (but it does not match after a newline at the very end of Haystack).

2) Dollar-sign ($) matches before any newlines in Haystack (as well as at the very end where it always matches).

For example, the pattern "m)^abc$" would not match the Haystack"xyz`r`nabc" unless the "m" option is present.

The "D" option is ignored when "m" is present.

An asterisk matches zero or more of the preceding character, class, or subpattern. For example, a* matches ab and aaab. It also matches at the very beginning of any string that contains no "a" at all.

Wildcard: The dot-star pattern .* is one of the most permissive because it matches zero or more occurrences of any character (except newline: `r and `n). For example, abc.*123 matches abcAnything123 as well as abc123.

Matches between min and max occurrences of the preceding character, class, or subpattern. For example, a{1,2} matches ab but only the first two a's in aaab.

Also, {3} means exactly 3 occurrences, and {3,} means 3 or more occurrences. Note: The specified numbers must be less than 65536, and the first must be less than or equal to the second.

Classes of characters: The square brackets enclose a list or range of characters (or both). For example, [abc] means "any single character that is either a, b or c". Using a dash in between creates a range; for example, [a-z] means "any single character that is between lowercase a and z (inclusive)". Lists and ranges may be combined; for example [a-zA-Z0-9_] means "any single character that is alphanumeric or underscore".

A character class may be followed by *, ?, +, or {min,max}. For example, [0-9]+ matches one or more occurrence of any digit; thus it matches xyz123 but not abcxyz.

The following POSIX named sets are also supported via the form [[:xxx:]], where xxx is one of the following words: alnum, alpha, ascii (0-127), blank (space or tab), cntrl (control character), digit (0-9), xdigit (hex digit), print, graph (print excluding space), punct, lower, upper, space (whitespace), word (same as \w).

Within a character class, characters do not need to be escaped except when they have special meaning inside a class; e.g. [\^a], [a\-b], [a\]], and [\\a].

Circumflex (^) and dollar sign ($) are called anchors because they don't consume any characters; instead, they tie the pattern to the beginning or end of the string being searched.

^ may appear at the beginning of a pattern to require the match to occur at the very beginning of a line. For example, ^abc matches abc123 but not 123abc.

$ may appear at the end of a pattern to require the match to occur at the very end of a line. For example, abc$ matches 123abc but not abc123.

The two anchors may be combined. For example, ^abc$ matches only abc (i.e. there must be no other characters before or after it).

If the text being searched contains multiple lines, the anchors can be made to apply to each line rather than the text as a whole by means of the "m" option. For example, m)^abc$ matches 123`r`nabc`r`n789. But without the "m" option, it wouldn't match.

Items enclosed in parentheses are most commonly used to:

• Determine the order of evaluation. For example, (Sun|Mon|Tues|Wednes|Thurs|Fri|Satur)day matches the name of any day.
• Apply *, ?, +, or {min,max} to a series of characters rather than just one. For example, (abc)+ matches one or more occurrences of the string "abc"; thus it matches abcabc123 but not ab123 or bc123.
• Capture a subpattern such as the dot-star in abc(.*)xyz. For example, RegExMatch() stores the substring that matches each subpattern in its output array. Similarly, RegExReplace() allows the substring that matches each subpattern to be reinserted into the result via backreferences like $1. To use the parentheses without the side-effect of capturing a subpattern, specify ?: as the first two characters inside the parentheses; for example: (?:.*)
• Change options on-the-fly. For example, (?im) turns on the case-insensitive and multiline options for the remainder of the pattern (or subpattern if it occurs inside a subpattern). Conversely, (?-im) would turn them both off. All options are supported except DPS`r`n`a.

These escape sequences stand for special characters. The most common ones are \t (tab), \r (carriage return), and \n (linefeed). In AutoHotkey, an accent (`) may optionally be used in place of the backslash in these cases. Escape sequences in the form \xhh are also supported, in which hh is the hex code of any ANSI character between 00 and FF.

[v1.0.46.06+]: \R means "any single newline of any type", namely those listed at the `a option (however, \R inside a character class is merely the letter "R"). [v1.0.47.05+]: \R can be restricted to CR, LF, and CRLF by specifying (*BSR_ANYCRLF) in uppercase at the beginning of the pattern (after the options); e.g. im)(*BSR_ANYCRLF)abc\Rxyz

[AHK_L 61+]: Unicode character properties. Not supported on ANSI builds. \p{xx} matches a character with the xx property while \P{xx} matches any character without the xx property. For example, \pL matches any letter and \p{Lu} matches any upper-case letter. \X matches any number of characters that form an extended Unicode sequence.

For a full list of supported property names and other details, search for "\p{xx}" at www.pcre.org/pcre.txt.

[AHK_L 61+]: For performance, \d, \D, \s, \S, \w, \W, \b and \B recognize only ASCII characters by default, even on Unicode builds. If the pattern begins with (*UCP), Unicode properties will be used to determine which characters match. For example, \w becomes equivalent to [\p{L}\p{N}_] and \d becomes equivalent to \p{Nd}.