scanner - The Go Programming Language


Package scanner

import "text/scanner"

Overview ?

Overview ?

Package scanner provides a scanner and tokenizer for UTF-8-encoded text. It takes an io.Reader providing the source, which then can be tokenized through repeated calls to the Scan function. For compatibility with existing tools, the NUL character is not allowed.

By default, a Scanner skips white space and Go comments and recognizes all literals as defined by the Go language specification. It may be customized to recognize only a subset of those literals and to recognize different white space characters.

Basic usage pattern:

var s scanner.Scanner
tok := s.Scan()
for tok != scanner.EOF {
	// do something with tok
	tok = s.Scan()


func TokenString(tok rune) string
type Position
    func (pos *Position) IsValid() bool
    func (pos Position) String() string
type Scanner
    func (s *Scanner) Init(src io.Reader) *Scanner
    func (s *Scanner) Next() rune
    func (s *Scanner) Peek() rune
    func (s *Scanner) Pos() (pos Position)
    func (s *Scanner) Scan() rune
    func (s *Scanner) TokenText() string

Package files



const (
    ScanIdents     = 1 << -Ident
    ScanInts       = 1 << -Int
    ScanFloats     = 1 << -Float // includes Ints
    ScanChars      = 1 << -Char
    ScanStrings    = 1 << -String
    ScanRawStrings = 1 << -RawString
    ScanComments   = 1 << -Comment
    SkipComments   = 1 << -skipComment // if set with ScanComments, comments become white space
    GoTokens       = ScanIdents | ScanFloats | ScanChars | ScanStrings | ScanRawStrings | ScanComments | SkipComments

Predefined mode bits to control recognition of tokens. For instance, to configure a Scanner such that it only recognizes (Go) identifiers, integers, and skips comments, set the Scanner's Mode field to:

ScanIdents | ScanInts | SkipComments
const (
    EOF = -(iota + 1)

The result of Scan is one of the following tokens or a Unicode character.

const GoWhitespace = 1<<'\t' | 1<<'\n' | 1<<'\r' | 1<<' '

GoWhitespace is the default value for the Scanner's Whitespace field. Its value selects Go's white space characters.

func TokenString

func TokenString(tok rune) string

TokenString returns a printable string for a token or Unicode character.

type Position

type Position struct {
    Filename string // filename, if any
    Offset   int    // byte offset, starting at 0
    Line     int    // line number, starting at 1
    Column   int    // column number, starting at 1 (character count per line)

A source position is represented by a Position value. A position is valid if Line > 0.

func (*Position) IsValid

func (pos *Position) IsValid() bool

IsValid returns true if the position is valid.

func (Position) String

func (pos Position) String() string

type Scanner

type Scanner struct {

    // Error is called for each error encountered. If no Error
    // function is set, the error is reported to os.Stderr.
    Error func(s *Scanner, msg string)

    // ErrorCount is incremented by one for each error encountered.
    ErrorCount int

    // The Mode field controls which tokens are recognized. For instance,
    // to recognize Ints, set the ScanInts bit in Mode. The field may be
    // changed at any time.
    Mode uint

    // The Whitespace field controls which characters are recognized
    // as white space. To recognize a character ch <= ' ' as white space,
    // set the ch'th bit in Whitespace (the Scanner's behavior is undefined
    // for values ch > ' '). The field may be changed at any time.
    Whitespace uint64

    // Start position of most recently scanned token; set by Scan.
    // Calling Init or Next invalidates the position (Line == 0).
    // The Filename field is always left untouched by the Scanner.
    // If an error is reported (via Error) and Position is invalid,
    // the scanner is not inside a token. Call Pos to obtain an error
    // position in that case.
    // contains filtered or unexported fields

A Scanner implements reading of Unicode characters and tokens from an io.Reader.

func (*Scanner) Init

func (s *Scanner) Init(src io.Reader) *Scanner

Init initializes a Scanner with a new source and returns s. Error is set to nil, ErrorCount is set to 0, Mode is set to GoTokens, and Whitespace is set to GoWhitespace.

func (*Scanner) Next

func (s *Scanner) Next() rune

Next reads and returns the next Unicode character. It returns EOF at the end of the source. It reports a read error by calling s.Error, if not nil; otherwise it prints an error message to os.Stderr. Next does not update the Scanner's Position field; use Pos() to get the current position.

func (*Scanner) Peek

func (s *Scanner) Peek() rune

Peek returns the next Unicode character in the source without advancing the scanner. It returns EOF if the scanner's position is at the last character of the source.

func (*Scanner) Pos

func (s *Scanner) Pos() (pos Position)

Pos returns the position of the character immediately after the character or token returned by the last call to Next or Scan.

func (*Scanner) Scan

func (s *Scanner) Scan() rune

Scan reads the next token or Unicode character from source and returns it. It only recognizes tokens t for which the respective Mode bit (1<<-t) is set. It returns EOF at the end of the source. It reports scanner errors (read and token errors) by calling s.Error, if not nil; otherwise it prints an error message to os.Stderr.

func (*Scanner) TokenText

func (s *Scanner) TokenText() string

TokenText returns the string corresponding to the most recently scanned token. Valid after calling Scan().