Package syntax parses regular expressions into parse trees and compiles parse trees into programs. Most clients of regular expressions will use the facilities of package regexp (such as Compile and Match) instead of this package.


The regular expression syntax understood by this package when parsing with the Perl flag is as follows. Parts of the syntax can be disabled by passing alternate flags to Parse.

Single characters:

.              any character, possibly including newline (flag s=true)
[xyz]          character class
[^xyz]         negated character class
\d             Perl character class
\D             negated Perl character class
[[:alpha:]]    ASCII character class
[[:^alpha:]]   negated ASCII character class
\pN            Unicode character class (one-letter name)
\p{Greek}      Unicode character class
\PN            negated Unicode character class (one-letter name)
\P{Greek}      negated Unicode character class


xy             x followed by y
x|y            x or y (prefer x)


x*             zero or more x, prefer more
x+             one or more x, prefer more
x?             zero or one x, prefer one
x{n,m}         n or n+1 or ... or m x, prefer more
x{n,}          n or more x, prefer more
x{n}           exactly n x
x*?            zero or more x, prefer fewer
x+?            one or more x, prefer fewer
x??            zero or one x, prefer zero
x{n,m}?        n or n+1 or ... or m x, prefer fewer
x{n,}?         n or more x, prefer fewer
x{n}?          exactly n x

Implementation restriction: The counting forms x{n,m}, x{n,}, and x{n} reject forms that create a minimum or maximum repetition count above 1000. Unlimited repetitions are not subject to this restriction.


(re)           numbered capturing group (submatch)
(?P<name>re)   named & numbered capturing group (submatch)
(?:re)         non-capturing group
(?flags)       set flags within current group; non-capturing
(?flags:re)    set flags during re; non-capturing

Flag syntax is xyz (set) or -xyz (clear) or xy-z (set xy, clear z). The flags are:

i              case-insensitive (default false)
m              multi-line mode: ^ and $ match begin/end line in addition to begin/end text (default false)
s              let . match \n (default false)
U              ungreedy: swap meaning of x* and x*?, x+ and x+?, etc (default false)

Empty strings:

^              at beginning of text or line (flag m=true)
$              at end of text (like \z not \Z) or line (flag m=true)
\A             at beginning of text
\b             at ASCII word boundary (\w on one side and \W, \A, or \z on the other)
\B             not at ASCII word boundary
\z             at end of text

Escape sequences:

\a             bell (== \007)
\f             form feed (== \014)
\t             horizontal tab (== \011)
\n             newline (== \012)
\r             carriage return (== \015)
\v             vertical tab character (== \013)
\*             literal *, for any punctuation character *
\123           octal character code (up to three digits)
\x7F           hex character code (exactly two digits)
\x{10FFFF}     hex character code
\Q...\E        literal text ... even if ... has punctuation

Character class elements:

x              single character
A-Z            character range (inclusive)
\d             Perl character class
[:foo:]        ASCII character class foo
\p{Foo}        Unicode character class Foo
\pF            Unicode character class F (one-letter name)

Named character classes as character class elements:

[\d]           digits (== \d)
[^\d]          not digits (== \D)
[\D]           not digits (== \D)
[^\D]          not not digits (== \d)
[[:name:]]     named ASCII class inside character class (== [:name:])
[^[:name:]]    named ASCII class inside negated character class (== [:^name:])
[\p{Name}]     named Unicode property inside character class (== \p{Name})
[^\p{Name}]    named Unicode property inside negated character class (== \P{Name})

Perl character classes (all ASCII-only):

\d             digits (== [0-9])
\D             not digits (== [^0-9])
\s             whitespace (== [\t\n\f\r ])
\S             not whitespace (== [^\t\n\f\r ])
\w             word characters (== [0-9A-Za-z_])
\W             not word characters (== [^0-9A-Za-z_])

ASCII character classes:

[[:alnum:]]    alphanumeric (== [0-9A-Za-z])
[[:alpha:]]    alphabetic (== [A-Za-z])
[[:ascii:]]    ASCII (== [\x00-\x7F])
[[:blank:]]    blank (== [\t ])
[[:cntrl:]]    control (== [\x00-\x1F\x7F])
[[:digit:]]    digits (== [0-9])
[[:graph:]]    graphical (== [!-~] == [A-Za-z0-9!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~])
[[:lower:]]    lower case (== [a-z])
[[:print:]]    printable (== [ -~] == [ [:graph:]])
[[:punct:]]    punctuation (== [!-/:-@[-`{-~])
[[:space:]]    whitespace (== [\t\n\v\f\r ])
[[:upper:]]    upper case (== [A-Z])
[[:word:]]     word characters (== [0-9A-Za-z_])
[[:xdigit:]]   hex digit (== [0-9A-Fa-f])

Unicode character classes are those in unicode.Categories and unicode.Scripts.



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func IsWordChar

func IsWordChar(r rune) bool

    IsWordChar reports whether r is consider a “word character” during the evaluation of the \b and \B zero-width assertions. These assertions are ASCII-only: the word characters are [A-Za-z0-9_].


    type EmptyOp

    type EmptyOp uint8

      An EmptyOp specifies a kind or mixture of zero-width assertions.

      const (
      	EmptyBeginLine EmptyOp = 1 << iota

      func EmptyOpContext

      func EmptyOpContext(r1, r2 rune) EmptyOp

        EmptyOpContext returns the zero-width assertions satisfied at the position between the runes r1 and r2. Passing r1 == -1 indicates that the position is at the beginning of the text. Passing r2 == -1 indicates that the position is at the end of the text.

        type Error

        type Error struct {
        	Code ErrorCode
        	Expr string

          An Error describes a failure to parse a regular expression and gives the offending expression.

          func (*Error) Error

          func (e *Error) Error() string

          type ErrorCode

          type ErrorCode string

            An ErrorCode describes a failure to parse a regular expression.

            const (
            	// Unexpected error
            	ErrInternalError ErrorCode = "regexp/syntax: internal error"
            	// Parse errors
            	ErrInvalidCharClass      ErrorCode = "invalid character class"
            	ErrInvalidCharRange      ErrorCode = "invalid character class range"
            	ErrInvalidEscape         ErrorCode = "invalid escape sequence"
            	ErrInvalidNamedCapture   ErrorCode = "invalid named capture"
            	ErrInvalidPerlOp         ErrorCode = "invalid or unsupported Perl syntax"
            	ErrInvalidRepeatOp       ErrorCode = "invalid nested repetition operator"
            	ErrInvalidRepeatSize     ErrorCode = "invalid repeat count"
            	ErrInvalidUTF8           ErrorCode = "invalid UTF-8"
            	ErrMissingBracket        ErrorCode = "missing closing ]"
            	ErrMissingParen          ErrorCode = "missing closing )"
            	ErrMissingRepeatArgument ErrorCode = "missing argument to repetition operator"
            	ErrTrailingBackslash     ErrorCode = "trailing backslash at end of expression"
            	ErrUnexpectedParen       ErrorCode = "unexpected )"

            func (ErrorCode) String

            func (e ErrorCode) String() string

            type Flags

            type Flags uint16

              Flags control the behavior of the parser and record information about regexp context.

              const (
              	FoldCase      Flags = 1 << iota // case-insensitive match
              	Literal                         // treat pattern as literal string
              	ClassNL                         // allow character classes like [^a-z] and [[:space:]] to match newline
              	DotNL                           // allow . to match newline
              	OneLine                         // treat ^ and $ as only matching at beginning and end of text
              	NonGreedy                       // make repetition operators default to non-greedy
              	PerlX                           // allow Perl extensions
              	UnicodeGroups                   // allow \p{Han}, \P{Han} for Unicode group and negation
              	WasDollar                       // regexp OpEndText was $, not \z
              	Simple                          // regexp contains no counted repetition
              	MatchNL = ClassNL | DotNL
              	Perl        = ClassNL | OneLine | PerlX | UnicodeGroups // as close to Perl as possible
              	POSIX Flags = 0                                         // POSIX syntax

              type Inst

              type Inst struct {
              	Op   InstOp
              	Out  uint32 // all but InstMatch, InstFail
              	Arg  uint32 // InstAlt, InstAltMatch, InstCapture, InstEmptyWidth
              	Rune []rune

                An Inst is a single instruction in a regular expression program.

                func (*Inst) MatchEmptyWidth

                func (i *Inst) MatchEmptyWidth(before rune, after rune) bool

                  MatchEmptyWidth reports whether the instruction matches an empty string between the runes before and after. It should only be called when i.Op == InstEmptyWidth.

                  func (*Inst) MatchRune

                  func (i *Inst) MatchRune(r rune) bool

                    MatchRune reports whether the instruction matches (and consumes) r. It should only be called when i.Op == InstRune.

                    func (*Inst) MatchRunePos

                    func (i *Inst) MatchRunePos(r rune) int

                      MatchRunePos checks whether the instruction matches (and consumes) r. If so, MatchRunePos returns the index of the matching rune pair (or, when len(i.Rune) == 1, rune singleton). If not, MatchRunePos returns -1. MatchRunePos should only be called when i.Op == InstRune.

                      func (*Inst) String

                      func (i *Inst) String() string

                      type InstOp

                      type InstOp uint8

                        An InstOp is an instruction opcode.

                        const (
                        	InstAlt InstOp = iota

                        func (InstOp) String

                        func (i InstOp) String() string

                        type Op

                        type Op uint8

                          An Op is a single regular expression operator.

                          const (
                          	OpNoMatch        Op = 1 + iota // matches no strings
                          	OpEmptyMatch                   // matches empty string
                          	OpLiteral                      // matches Runes sequence
                          	OpCharClass                    // matches Runes interpreted as range pair list
                          	OpAnyCharNotNL                 // matches any character except newline
                          	OpAnyChar                      // matches any character
                          	OpBeginLine                    // matches empty string at beginning of line
                          	OpEndLine                      // matches empty string at end of line
                          	OpBeginText                    // matches empty string at beginning of text
                          	OpEndText                      // matches empty string at end of text
                          	OpWordBoundary                 // matches word boundary `\b`
                          	OpNoWordBoundary               // matches word non-boundary `\B`
                          	OpCapture                      // capturing subexpression with index Cap, optional name Name
                          	OpStar                         // matches Sub[0] zero or more times
                          	OpPlus                         // matches Sub[0] one or more times
                          	OpQuest                        // matches Sub[0] zero or one times
                          	OpRepeat                       // matches Sub[0] at least Min times, at most Max (Max == -1 is no limit)
                          	OpConcat                       // matches concatenation of Subs
                          	OpAlternate                    // matches alternation of Subs

                          func (Op) String

                          func (i Op) String() string

                          type Prog

                          type Prog struct {
                          	Inst   []Inst
                          	Start  int // index of start instruction
                          	NumCap int // number of InstCapture insts in re

                            A Prog is a compiled regular expression program.

                            func Compile

                            func Compile(re *Regexp) (*Prog, error)

                              Compile compiles the regexp into a program to be executed. The regexp should have been simplified already (returned from re.Simplify).

                              func (*Prog) Prefix

                              func (p *Prog) Prefix() (prefix string, complete bool)

                                Prefix returns a literal string that all matches for the regexp must start with. Complete is true if the prefix is the entire match.

                                func (*Prog) StartCond

                                func (p *Prog) StartCond() EmptyOp

                                  StartCond returns the leading empty-width conditions that must be true in any match. It returns ^EmptyOp(0) if no matches are possible.

                                  func (*Prog) String

                                  func (p *Prog) String() string

                                  type Regexp

                                  type Regexp struct {
                                  	Op       Op // operator
                                  	Flags    Flags
                                  	Sub      []*Regexp  // subexpressions, if any
                                  	Sub0     [1]*Regexp // storage for short Sub
                                  	Rune     []rune     // matched runes, for OpLiteral, OpCharClass
                                  	Rune0    [2]rune    // storage for short Rune
                                  	Min, Max int        // min, max for OpRepeat
                                  	Cap      int        // capturing index, for OpCapture
                                  	Name     string     // capturing name, for OpCapture

                                    A Regexp is a node in a regular expression syntax tree.

                                    func Parse

                                    func Parse(s string, flags Flags) (*Regexp, error)

                                      Parse parses a regular expression string s, controlled by the specified Flags, and returns a regular expression parse tree. The syntax is described in the top-level comment.

                                      func (*Regexp) CapNames

                                      func (re *Regexp) CapNames() []string

                                        CapNames walks the regexp to find the names of capturing groups.

                                        func (*Regexp) Equal

                                        func (x *Regexp) Equal(y *Regexp) bool

                                          Equal reports whether x and y have identical structure.

                                          func (*Regexp) MaxCap

                                          func (re *Regexp) MaxCap() int

                                            MaxCap walks the regexp to find the maximum capture index.

                                            func (*Regexp) Simplify

                                            func (re *Regexp) Simplify() *Regexp

                                              Simplify returns a regexp equivalent to re but without counted repetitions and with various other simplifications, such as rewriting /(?:a+)+/ to /a+/. The resulting regexp will execute correctly but its string representation will not produce the same parse tree, because capturing parentheses may have been duplicated or removed. For example, the simplified form for /(x){1,2}/ is /(x)(x)?/ but both parentheses capture as $1. The returned regexp may share structure with or be the original.

                                              func (*Regexp) String

                                              func (re *Regexp) String() string