regexp

package standard library
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 Go to latest Published: Apr 16, 2026 License: BSD-3-Clause Imports: 10 Imported by: 507,717

Documentation

Overview

Package regexp implements regular expression search.

The syntax of the regular expressions accepted is the same general syntax used by Perl, Python, and other languages. More precisely, it is the syntax accepted by RE2 and described at https://golang.org/s/re2syntax, except for \C. For an overview of the syntax, see the regexp/syntax package.

The regexp implementation provided by this package is guaranteed to run in time linear in the size of the input. (This is a property not guaranteed by most open source implementations of regular expressions.) For more information about this property, see https://swtch.com/~rsc/regexp/regexp1.html or any book about automata theory.

All characters are UTF-8-encoded code points. Following utf8.DecodeRune, each byte of an invalid UTF-8 sequence is treated as if it encoded utf8.RuneError (U+FFFD).

There are 24 methods of Regexp that match a regular expression and identify the matched text. Their names are matched by this regular expression: 

(All|Find|FindAll)(String)?(Submatch)?(Index)?

The ‘All’ variants return an iterator over successive non-overlapping matches of the entire expression. The ‘FindAll’ variants return a slice of those matches instead. Empty matches abutting a preceding match are ignored. The ‘FindAll’ variants take an extra integer argument, n. If n >= 0, the function returns at most n matches/submatches; otherwise, it returns all of them.

The ‘Find’ variants return only the first match that All or FindAll would return.

If ‘String’ is present, the argument is a string; otherwise it is a []byte.

By default, each returned match is denoted by the substring matching the regular expression, of type string or []byte according to the type of the argument. If ‘Submatch’ is present, each match is represented instead by a slice of the substrings matching the regular expression's parenthesized subexpressions (also known as capturing groups), numbered from left to right in order of opening parenthesis. Submatch 0 is the match of the entire expression, submatch 1 is the match of the first parenthesized subexpression, and so on. If ‘Index’ is present, each substring is instead denoted by a pair of byte indexes within the input string. If an index is negative or substring is nil, it means that the subexpression did not match any string in the input. For ‘String’ versions, an empty string means either no match or an empty match.

There is also a subset of the methods that can be applied to text read from an io.RuneReader: Regexp.MatchReader, Regexp.FindReaderIndex, Regexp.FindReaderSubmatchIndex. Note that regular expression matches may need to examine text beyond the text returned by a match, so the methods that match text from an io.RuneReader may read arbitrarily far into the input before returning.

(There are a few other methods that do not match this pattern.)

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	// Compile the expression once, usually at init time.
	// Use raw strings to avoid having to quote the backslashes.
	var validID = regexp.MustCompile(`^[a-z]+\[[0-9]+\]$`)

	fmt.Println(validID.MatchString("adam[23]"))
	fmt.Println(validID.MatchString("eve[7]"))
	fmt.Println(validID.MatchString("Job[48]"))
	fmt.Println(validID.MatchString("snakey"))
}

Output:
true
true
false
false

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

func Match 

func Match(pattern string, b []byte) (matched bool, err error)

Match reports whether the byte slice b contains any match of the regular expression pattern. More complicated queries need to use Compile and the full Regexp interface.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	matched, err := regexp.Match(`foo.*`, []byte(`seafood`))
	fmt.Println(matched, err)
	matched, err = regexp.Match(`bar.*`, []byte(`seafood`))
	fmt.Println(matched, err)
	matched, err = regexp.Match(`a(b`, []byte(`seafood`))
	fmt.Println(matched, err)

}

Output:
true <nil>
false <nil>
false error parsing regexp: missing closing ): `a(b`

func MatchReader 

func MatchReader(pattern string, r io.RuneReader) (matched bool, err error)

MatchReader reports whether the text returned by the io.RuneReader contains any match of the regular expression pattern. More complicated queries need to use Compile and the full Regexp interface.

func MatchString 

func MatchString(pattern string, s string) (matched bool, err error)

MatchString reports whether the string s contains any match of the regular expression pattern. More complicated queries need to use Compile and the full Regexp interface.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	matched, err := regexp.MatchString(`foo.*`, "seafood")
	fmt.Println(matched, err)
	matched, err = regexp.MatchString(`bar.*`, "seafood")
	fmt.Println(matched, err)
	matched, err = regexp.MatchString(`a(b`, "seafood")
	fmt.Println(matched, err)
}

Output:
true <nil>
false <nil>
false error parsing regexp: missing closing ): `a(b`

func QuoteMeta 

func QuoteMeta(s string) string

QuoteMeta returns a string that escapes all regular expression metacharacters inside the argument text; the returned string is a regular expression matching the literal text.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	fmt.Println(regexp.QuoteMeta(`Escaping symbols like: .+*?()|[]{}^$`))
}

Output:
Escaping symbols like: \.\+\*\?\(\)\|\[\]\{\}\^\$

Types

type Regexp 

type Regexp struct {
	// contains filtered or unexported fields
}

Regexp is the representation of a compiled regular expression. A Regexp is safe for concurrent use by multiple goroutines, except for configuration methods, such as Regexp.Longest.

func Compile 

func Compile(expr string) (*Regexp, error)

Compile parses a regular expression and returns, if successful, a Regexp object that can be used to match against text.

When matching against text, the regexp returns a match that begins as early as possible in the input (leftmost), and among those it chooses the one that a backtracking search would have found first. This so-called leftmost-first matching is the same semantics that Perl, Python, and other implementations use, although this package implements it without the expense of backtracking. For POSIX leftmost-longest matching, see CompilePOSIX.

func CompilePOSIX 

func CompilePOSIX(expr string) (*Regexp, error)

CompilePOSIX is like Compile but restricts the regular expression to POSIX ERE (egrep) syntax and changes the match semantics to leftmost-longest.

That is, when matching against text, the regexp returns a match that begins as early as possible in the input (leftmost), and among those it chooses a match that is as long as possible. This so-called leftmost-longest matching is the same semantics that early regular expression implementations used and that POSIX specifies.

However, there can be multiple leftmost-longest matches, with different submatch choices, and here this package diverges from POSIX. Among the possible leftmost-longest matches, this package chooses the one that a backtracking search would have found first, while POSIX specifies that the match be chosen to maximize the length of the first subexpression, then the second, and so on from left to right. The POSIX rule is computationally prohibitive and not even well-defined. See https://swtch.com/~rsc/regexp/regexp2.html#posix for details.

func MustCompile 

func MustCompile(str string) *Regexp

MustCompile is like Compile but panics if the expression cannot be parsed. It simplifies safe initialization of global variables holding compiled regular expressions.

func MustCompilePOSIX 

func MustCompilePOSIX(str string) *Regexp

MustCompilePOSIX is like CompilePOSIX but panics if the expression cannot be parsed. It simplifies safe initialization of global variables holding compiled regular expressions.

func (*Regexp) AppendText added in go1.24.0 

func (re *Regexp) AppendText(b []byte) ([]byte, error)

AppendText implements encoding.TextAppender. The output matches that of calling the Regexp.String method.

Note that the output is lossy in some cases: This method does not indicate POSIX regular expressions (i.e. those compiled by calling CompilePOSIX), or those for which the Regexp.Longest method has been called.

func (*Regexp) Copy deprecated added in go1.6 

func (re *Regexp) Copy() *Regexp

Copy returns a new Regexp object copied from re. Calling Regexp.Longest on one copy does not affect another.

Deprecated: In earlier releases, when using a Regexp in multiple goroutines, giving each goroutine its own copy helped to avoid lock contention. As of Go 1.12, using Copy is no longer necessary to avoid lock contention. Copy may still be appropriate if the reason for its use is to make two copies with different Regexp.Longest settings.

func (*Regexp) Expand 

func (re *Regexp) Expand(dst []byte, template []byte, src []byte, match []int) []byte

Expand appends template to dst and returns the result; during the append, Expand replaces variables in the template with corresponding matches drawn from src. The match slice should have been returned by Regexp.FindSubmatchIndex.

In the template, a variable is denoted by a substring of the form $name or ${name}, where name is a non-empty sequence of letters, digits, and underscores. A purely numeric name like $1 refers to the submatch with the corresponding index; other names refer to capturing parentheses named with the (?P<name>...) syntax. A reference to an out of range or unmatched index or a name that is not present in the regular expression is replaced with an empty slice.

In the $name form, name is taken to be as long as possible: $1x is equivalent to ${1x}, not ${1}x, and, $10 is equivalent to ${10}, not ${1}0.

To insert a literal $ in the output, use $$ in the template.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	content := []byte(`
	# comment line
	option1: value1
	option2: value2

	# another comment line
	option3: value3
`)

	// Regex pattern captures "key: value" pair from the content.
	pattern := regexp.MustCompile(`(?m)(?P<key>\w+):\s+(?P<value>\w+)$`)

	// Template to convert "key: value" to "key=value" by
	// referencing the values captured by the regex pattern.
	template := []byte("$key=$value\n")

	result := []byte{}

	// For each match of the regex in the content.
	for _, submatches := range pattern.FindAllSubmatchIndex(content, -1) {
		// Apply the captured submatches to the template and append the output
		// to the result.
		result = pattern.Expand(result, template, content, submatches)
	}
	fmt.Println(string(result))
}

Output:
option1=value1
option2=value2
option3=value3

func (*Regexp) ExpandString 

func (re *Regexp) ExpandString(dst []byte, template string, src string, match []int) []byte

ExpandString is like Regexp.Expand but the template and source are strings. It appends to and returns a byte slice in order to give the calling code control over allocation.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	content := `
	# comment line
	option1: value1
	option2: value2

	# another comment line
	option3: value3
`

	// Regex pattern captures "key: value" pair from the content.
	pattern := regexp.MustCompile(`(?m)(?P<key>\w+):\s+(?P<value>\w+)$`)

	// Template to convert "key: value" to "key=value" by
	// referencing the values captured by the regex pattern.
	template := "$key=$value\n"

	result := []byte{}

	// For each match of the regex in the content.
	for _, submatches := range pattern.FindAllStringSubmatchIndex(content, -1) {
		// Apply the captured submatches to the template and append the output
		// to the result.
		result = pattern.ExpandString(result, template, content, submatches)
	}
	fmt.Println(string(result))
}

Output:
option1=value1
option2=value2
option3=value3

func (*Regexp) Find 

func (re *Regexp) Find(b []byte) []byte

Find returns the text of the leftmost match for re in b. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo.?`)
	fmt.Printf("%q\n", re.Find([]byte(`seafood fool`)))

}

Output:
"food"

func (*Regexp) FindAll 

func (re *Regexp) FindAll(b []byte, n int) [][]byte

FindAll returns all the matches for re in b. If n >= 0, FindAll returns no more than n matches. See [Regexp.All] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo.?`)
	fmt.Printf("%q\n", re.FindAll([]byte(`seafood fool`), -1))

}

Output:
["food" "fool"]

func (*Regexp) FindAllIndex 

func (re *Regexp) FindAllIndex(b []byte, n int) [][]int

FindAllIndex returns the locations of all matches for re in b. If n >= 0, FindAllIndex returns no more than n matches. See [Regexp.AllIndex] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	content := []byte("London")
	re := regexp.MustCompile(`o.`)
	fmt.Println(re.FindAllIndex(content, 1))
	fmt.Println(re.FindAllIndex(content, -1))
}

Output:
[[1 3]]
[[1 3] [4 6]]

func (*Regexp) FindAllString 

func (re *Regexp) FindAllString(s string, n int) []string

FindAllString returns all the matches for re in s. If n >= 0, FindAllString returns no more than n matches. See [Regexp.AllString] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a.`)
	fmt.Println(re.FindAllString("paranormal", -1))
	fmt.Println(re.FindAllString("paranormal", 2))
	fmt.Println(re.FindAllString("graal", -1))
	fmt.Println(re.FindAllString("none", -1))
}

Output:
[ar an al]
[ar an]
[aa]
[]

func (*Regexp) FindAllStringIndex 

func (re *Regexp) FindAllStringIndex(s string, n int) [][]int

FindAllStringIndex returns the locations of all matches for re in s. If n >= 0, FindAllStringIndex returns no more than n matches. See [Regexp.AllStringIndex] for the equivalent iterator form.

func (*Regexp) FindAllStringSubmatch 

func (re *Regexp) FindAllStringSubmatch(s string, n int) [][]string

FindAllStringSubmatch returns the locations of all matches for re in s, including submatch locations. In each returned match m, m[0] is the overall match, m[1] is the first submatch, and so on. If n >= 0, FindAllStringSubmatch returns no more than n matches. See [Regexp.AllStringSubmatch] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	fmt.Printf("%q\n", re.FindAllStringSubmatch("-ab-", -1))
	fmt.Printf("%q\n", re.FindAllStringSubmatch("-axxb-", -1))
	fmt.Printf("%q\n", re.FindAllStringSubmatch("-ab-axb-", -1))
	fmt.Printf("%q\n", re.FindAllStringSubmatch("-axxb-ab-", -1))
}

Output:
[["ab" ""]]
[["axxb" "xx"]]
[["ab" ""] ["axb" "x"]]
[["axxb" "xx"] ["ab" ""]]

func (*Regexp) FindAllStringSubmatchIndex 

func (re *Regexp) FindAllStringSubmatchIndex(s string, n int) [][]int

FindAllStringSubmatchIndex returns the locations of all matches for re in s, including submatch locations. In each returned match m, the overall match is s[m[0]:m[1]], the first submatch is s[m[2]:m[3]], and so on. If n >= 0, FindAllStringSubmatchIndex returns no more than n matches. See [Regexp.AllStringSubmatchIndex] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	// Indices:
	//    01234567   012345678
	//    -ab-axb-   -axxb-ab-
	fmt.Println(re.FindAllStringSubmatchIndex("-ab-", -1))
	fmt.Println(re.FindAllStringSubmatchIndex("-axxb-", -1))
	fmt.Println(re.FindAllStringSubmatchIndex("-ab-axb-", -1))
	fmt.Println(re.FindAllStringSubmatchIndex("-axxb-ab-", -1))
	fmt.Println(re.FindAllStringSubmatchIndex("-foo-", -1))
}

Output:
[[1 3 2 2]]
[[1 5 2 4]]
[[1 3 2 2] [4 7 5 6]]
[[1 5 2 4] [6 8 7 7]]
[]

func (*Regexp) FindAllSubmatch 

func (re *Regexp) FindAllSubmatch(b []byte, n int) [][][]byte

FindAllSubmatch returns the locations of all matches for re in b, including submatch locations. In each returned match m, the overall match is m[0], the first submatch is m[1], and so on. If n >= 0, FindAllSubmatch returns no more than n matches. See [Regexp.AllSubmatch] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo(.?)`)
	fmt.Printf("%q\n", re.FindAllSubmatch([]byte(`seafood fool`), -1))

}

Output:
[["food" "d"] ["fool" "l"]]

func (*Regexp) FindAllSubmatchIndex 

func (re *Regexp) FindAllSubmatchIndex(b []byte, n int) [][]int

FindAllSubmatchIndex returns the locations of all matches for re in b, including submatch locations. In each returned match m, the overall match is b[m[0]:m[1]], the first submatch is b[m[2]:m[3]], and so on. If n >= 0, FindAllSubmatchIndex returns no more than n matches. See [Regexp.AllSubmatchIndex] for the equivalent iterator form.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	content := []byte(`
	# comment line
	option1: value1
	option2: value2
`)
	// Regex pattern captures "key: value" pair from the content.
	pattern := regexp.MustCompile(`(?m)(?P<key>\w+):\s+(?P<value>\w+)$`)
	allIndexes := pattern.FindAllSubmatchIndex(content, -1)
	for _, loc := range allIndexes {
		fmt.Println(loc)
		fmt.Println(string(content[loc[0]:loc[1]]))
		fmt.Println(string(content[loc[2]:loc[3]]))
		fmt.Println(string(content[loc[4]:loc[5]]))
	}
}

Output:
[18 33 18 25 27 33]
option1: value1
option1
value1
[35 50 35 42 44 50]
option2: value2
option2
value2

func (*Regexp) FindIndex 

func (re *Regexp) FindIndex(b []byte) (m []int)

FindIndex returns the location of the leftmost match for re in b. The match itself is at b[m[0]:m[1]]. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	content := []byte(`
	# comment line
	option1: value1
	option2: value2
`)
	// Regex pattern captures "key: value" pair from the content.
	pattern := regexp.MustCompile(`(?m)(?P<key>\w+):\s+(?P<value>\w+)$`)

	loc := pattern.FindIndex(content)
	fmt.Println(loc)
	fmt.Println(string(content[loc[0]:loc[1]]))
}

Output:
[18 33]
option1: value1

func (*Regexp) FindReaderIndex 

func (re *Regexp) FindReaderIndex(r io.RuneReader) (m []int)

FindReaderIndex returns the location of the leftmost match for re in r. The match starts at byte index m[0] and ends just before byte index m[1]. The return value is nil for no match.

FindReaderIndex may read arbitrarily far from r, including reading beyond the returned match.

func (*Regexp) FindReaderSubmatchIndex 

func (re *Regexp) FindReaderSubmatchIndex(r io.RuneReader) []int

FindReaderSubmatchIndex returns the first match for re in r, including submatches. The overall match is at byte index m[0] up to m[1], the first submatch is at byte index m[2] up to m[3], and so on. The return value is nil for no match.

FindReaderSubmatchIndex may read arbitrarily far from r, including reading beyond the returned match.

func (*Regexp) FindString 

func (re *Regexp) FindString(s string) string

FindString returns the text of the leftmost match for re in s. The return value is the empty string both for an empty match and for no match. To distinguish those two cases, use Regexp.FindStringIndex or Regexp.FindStringSubmatch.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo.?`)
	fmt.Printf("%q\n", re.FindString("seafood fool"))
	fmt.Printf("%q\n", re.FindString("meat"))
}

Output:
"food"
""

func (*Regexp) FindStringIndex 

func (re *Regexp) FindStringIndex(s string) (m []int)

FindStringIndex returns the location of the leftmost match for re in s. The match itself is at s[m[0]:m[1]]. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`ab?`)
	fmt.Println(re.FindStringIndex("tablett"))
	fmt.Println(re.FindStringIndex("foo") == nil)
}

Output:
[1 3]
true

func (*Regexp) FindStringSubmatch 

func (re *Regexp) FindStringSubmatch(s string) []string

FindStringSubmatch returns the first match for re in s, including submatches. The overall match is s[0], the first submatch is s[1], and so on. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b(y|z)c`)
	fmt.Printf("%q\n", re.FindStringSubmatch("-axxxbyc-"))
	fmt.Printf("%q\n", re.FindStringSubmatch("-abzc-"))
}

Output:
["axxxbyc" "xxx" "y"]
["abzc" "" "z"]

func (*Regexp) FindStringSubmatchIndex 

func (re *Regexp) FindStringSubmatchIndex(s string) []int

FindStringSubmatchIndex returns the first match for re in s, including submatches. The overall match is s[m[0]:m[1]], the first submatch is s[m[2]:m[3]], and so on. The return value is nil for no match.

func (*Regexp) FindSubmatch 

func (re *Regexp) FindSubmatch(b []byte) [][]byte

FindSubmatch returns the first match for re in b, including submatches. The overall match is m[0], the first submatch is m[1], and so on. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo(.?)`)
	fmt.Printf("%q\n", re.FindSubmatch([]byte(`seafood fool`)))

}

Output:
["food" "d"]

func (*Regexp) FindSubmatchIndex 

func (re *Regexp) FindSubmatchIndex(b []byte) []int

FindSubmatchIndex returns the first match for re in b, including submatches. The overall match is b[m[0]:m[1]], the first submatch is b[m[2]:m[3]], and so on. The return value is nil for no match.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	// Indices:
	//    01234567   012345678
	//    -ab-axb-   -axxb-ab-
	fmt.Println(re.FindSubmatchIndex([]byte("-ab-")))
	fmt.Println(re.FindSubmatchIndex([]byte("-axxb-")))
	fmt.Println(re.FindSubmatchIndex([]byte("-ab-axb-")))
	fmt.Println(re.FindSubmatchIndex([]byte("-axxb-ab-")))
	fmt.Println(re.FindSubmatchIndex([]byte("-foo-")))
}

Output:
[1 3 2 2]
[1 5 2 4]
[1 3 2 2]
[1 5 2 4]
[]

func (*Regexp) LiteralPrefix 

func (re *Regexp) LiteralPrefix() (prefix string, complete bool)

LiteralPrefix returns a literal string that must begin any match of the regular expression re. It returns the boolean true if the literal string comprises the entire regular expression.

func (*Regexp) Longest added in go1.1 

func (re *Regexp) Longest()

Longest makes future searches prefer the leftmost-longest match. That is, when matching against text, the regexp returns a match that begins as early as possible in the input (leftmost), and among those it chooses a match that is as long as possible. This method modifies the Regexp and may not be called concurrently with any other methods.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(|b)`)
	fmt.Println(re.FindString("ab"))
	re.Longest()
	fmt.Println(re.FindString("ab"))
}

Output:
a
ab

func (*Regexp) MarshalText added in go1.21.0 

func (re *Regexp) MarshalText() ([]byte, error)

MarshalText implements encoding.TextMarshaler. The output matches that of calling the Regexp.AppendText method.

See Regexp.AppendText for more information.

func (*Regexp) Match 

func (re *Regexp) Match(b []byte) bool

Match reports whether the byte slice b contains any match of the regular expression re.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`foo.?`)
	fmt.Println(re.Match([]byte(`seafood fool`)))
	fmt.Println(re.Match([]byte(`something else`)))

}

Output:
true
false

func (*Regexp) MatchReader 

func (re *Regexp) MatchReader(r io.RuneReader) bool

MatchReader reports whether the text returned by the io.RuneReader contains any match of the regular expression re.

func (*Regexp) MatchString 

func (re *Regexp) MatchString(s string) bool

MatchString reports whether the string s contains any match of the regular expression re.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`(gopher){2}`)
	fmt.Println(re.MatchString("gopher"))
	fmt.Println(re.MatchString("gophergopher"))
	fmt.Println(re.MatchString("gophergophergopher"))
}

Output:
false
true
true

func (*Regexp) NumSubexp 

func (re *Regexp) NumSubexp() int

NumSubexp returns the number of parenthesized subexpressions in this Regexp.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re0 := regexp.MustCompile(`a.`)
	fmt.Printf("%d\n", re0.NumSubexp())

	re := regexp.MustCompile(`(.*)((a)b)(.*)a`)
	fmt.Println(re.NumSubexp())
}

Output:
0
4

func (*Regexp) ReplaceAll 

func (re *Regexp) ReplaceAll(src, repl []byte) []byte

ReplaceAll returns a copy of src, replacing matches of the Regexp with the replacement text repl. Inside repl, $ signs are interpreted as in Regexp.Expand.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("T")))
	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("$1")))
	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("$1W")))
	fmt.Printf("%s\n", re.ReplaceAll([]byte("-ab-axxb-"), []byte("${1}W")))

	re2 := regexp.MustCompile(`a(?P<1W>x*)b`)
	fmt.Printf("%s\n", re2.ReplaceAll([]byte("-ab-axxb-"), []byte("$1W")))
	fmt.Printf("%s\n", re2.ReplaceAll([]byte("-ab-axxb-"), []byte("${1}W")))

}

Output:
-T-T-
--xx-
---
-W-xxW-
--xx-
-W-xxW-

func (*Regexp) ReplaceAllFunc 

func (re *Regexp) ReplaceAllFunc(src []byte, repl func([]byte) []byte) []byte

ReplaceAllFunc returns a copy of src in which all matches of the Regexp have been replaced by the return value of function repl applied to the matched byte slice. The replacement returned by repl is substituted directly, without using Regexp.Expand.

func (*Regexp) ReplaceAllLiteral 

func (re *Regexp) ReplaceAllLiteral(src, repl []byte) []byte

ReplaceAllLiteral returns a copy of src, replacing matches of the Regexp with the replacement bytes repl. The replacement repl is substituted directly, without using Regexp.Expand.

func (*Regexp) ReplaceAllLiteralString 

func (re *Regexp) ReplaceAllLiteralString(src, repl string) string

ReplaceAllLiteralString returns a copy of src, replacing matches of the Regexp with the replacement string repl. The replacement repl is substituted directly, without using Regexp.Expand.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	fmt.Println(re.ReplaceAllLiteralString("-ab-axxb-", "T"))
	fmt.Println(re.ReplaceAllLiteralString("-ab-axxb-", "$1"))
	fmt.Println(re.ReplaceAllLiteralString("-ab-axxb-", "${1}"))
}

Output:
-T-T-
-$1-$1-
-${1}-${1}-

func (*Regexp) ReplaceAllString 

func (re *Regexp) ReplaceAllString(src, repl string) string

ReplaceAllString returns a copy of src, replacing matches of the Regexp with the replacement string repl. Inside repl, $ signs are interpreted as in Regexp.Expand.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`a(x*)b`)
	fmt.Println(re.ReplaceAllString("-ab-axxb-", "T"))
	fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1"))
	fmt.Println(re.ReplaceAllString("-ab-axxb-", "$1W"))
	fmt.Println(re.ReplaceAllString("-ab-axxb-", "${1}W"))

	re2 := regexp.MustCompile(`a(?P<1W>x*)b`)
	fmt.Printf("%s\n", re2.ReplaceAllString("-ab-axxb-", "$1W"))
	fmt.Println(re.ReplaceAllString("-ab-axxb-", "${1}W"))

}

Output:
-T-T-
--xx-
---
-W-xxW-
--xx-
-W-xxW-

func (*Regexp) ReplaceAllStringFunc 

func (re *Regexp) ReplaceAllStringFunc(src string, repl func(string) string) string

ReplaceAllStringFunc returns a copy of src in which all matches of the Regexp have been replaced by the return value of function repl applied to the matched substring. The replacement returned by repl is substituted directly, without using Regexp.Expand.

Example 
package main

import (
	"fmt"
	"regexp"
	"strings"
)

func main() {
	re := regexp.MustCompile(`[^aeiou]`)
	fmt.Println(re.ReplaceAllStringFunc("seafood fool", strings.ToUpper))
}

Output:
SeaFooD FooL

func (*Regexp) Split added in go1.1 

func (re *Regexp) Split(s string, n int) []string

Split slices s into substrings separated by the expression and returns a slice of the substrings between those expression matches.

The slice returned by this method consists of all the substrings of s not contained in the slice returned by Regexp.FindAllString. When called on an expression that contains no metacharacters, it is equivalent to strings.SplitN.

Example: 

s := regexp.MustCompile("a*").Split("abaabaccadaaae", 5)
// s: ["", "b", "b", "c", "cadaaae"]

The count determines the number of substrings to return:

  • n > 0: at most n substrings; the last substring will be the unsplit remainder;
  • n == 0: the result is nil (zero substrings);
  • n < 0: all substrings.
Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	a := regexp.MustCompile(`a`)
	fmt.Println(a.Split("banana", -1))
	fmt.Println(a.Split("banana", 0))
	fmt.Println(a.Split("banana", 1))
	fmt.Println(a.Split("banana", 2))
	zp := regexp.MustCompile(`z+`)
	fmt.Println(zp.Split("pizza", -1))
	fmt.Println(zp.Split("pizza", 0))
	fmt.Println(zp.Split("pizza", 1))
	fmt.Println(zp.Split("pizza", 2))
}

Output:
[b n n ]
[]
[banana]
[b nana]
[pi a]
[]
[pizza]
[pi a]

func (*Regexp) String 

func (re *Regexp) String() string

String returns the source text used to compile the regular expression.

func (*Regexp) SubexpIndex added in go1.15 

func (re *Regexp) SubexpIndex(name string) int

SubexpIndex returns the index of the first subexpression with the given name, or -1 if there is no subexpression with that name.

Note that multiple subexpressions can be written using the same name, as in (?P<bob>a+)(?P<bob>b+), which declares two subexpressions named "bob". In this case, SubexpIndex returns the index of the leftmost such subexpression in the regular expression.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`(?P<first>[a-zA-Z]+) (?P<last>[a-zA-Z]+)`)
	fmt.Println(re.MatchString("Alan Turing"))
	matches := re.FindStringSubmatch("Alan Turing")
	lastIndex := re.SubexpIndex("last")
	fmt.Printf("last => %d\n", lastIndex)
	fmt.Println(matches[lastIndex])
}

Output:
true
last => 2
Turing

func (*Regexp) SubexpNames 

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

SubexpNames returns the names of the parenthesized subexpressions in this Regexp. The name for the first sub-expression is names[1], so that if m is a match slice, the name for m[i] is SubexpNames()[i]. Since the Regexp as a whole cannot be named, names[0] is always the empty string. The slice should not be modified.

Example 
package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`(?P<first>[a-zA-Z]+) (?P<last>[a-zA-Z]+)`)
	fmt.Println(re.MatchString("Alan Turing"))
	fmt.Printf("%q\n", re.SubexpNames())
	reversed := fmt.Sprintf("${%s} ${%s}", re.SubexpNames()[2], re.SubexpNames()[1])
	fmt.Println(reversed)
	fmt.Println(re.ReplaceAllString("Alan Turing", reversed))
}

Output:
true
["" "first" "last"]
${last} ${first}
Turing Alan

func (*Regexp) UnmarshalText added in go1.21.0 

func (re *Regexp) UnmarshalText(text []byte) error

UnmarshalText implements encoding.TextUnmarshaler by calling Compile on the encoded value.

Source Files

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Directories

Path Synopsis
Package syntax parses regular expressions into parse trees and compiles parse trees into programs.
 Package syntax parses regular expressions into parse trees and compiles parse trees into programs.

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