Run Format

Package syntax

import "regexp/syntax"

Overview ▾

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 Perl's \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])

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

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

type EmptyOp uint8
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

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

type Error struct {
        Code ErrorCode
        Expr string

func (*Error) Error

func (e *Error) Error() string

type ErrorCode

An ErrorCode describes a failure to parse a regular expression.

type ErrorCode string
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

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

type Flags uint16
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

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

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

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

An InstOp is an instruction opcode.

type InstOp uint8
const (
        InstAlt InstOp = iota

func (InstOp) String

func (i InstOp) String() string

type Op

An Op is a single regular expression operator.

type Op uint8
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

type Prog

A Prog is a compiled regular expression program.

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

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

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

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

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 returns true if 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