dlclark / regexp2

A full-featured regex engine in pure Go based on the .NET engine

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regexp2 - full featured regular expressions for Go

Regexp2 is a feature-rich RegExp engine for Go. It doesn't have constant time guarantees like the built-in regexp package, but it allows backtracking and is compatible with Perl5 and .NET. You'll likely be better off with the RE2 engine from the regexp package and should only use this if you need to write very complex patterns or require compatibility with .NET.

Basis of the engine

The engine is ported from the .NET framework's System.Text.RegularExpressions.Regex engine. That engine was open sourced in 2015 under the MIT license. There are some fundamental differences between .NET strings and Go strings that required a bit of borrowing from the Go framework regex engine as well. I cleaned up a couple of the dirtier bits during the port (regexcharclass.cs was terrible), but the parse tree, code emmitted, and therefore patterns matched should be identical.


This is a go-gettable library, so install is easy:

go get github.com/dlclark/regexp2/...


Usage is similar to the Go regexp package. Just like in regexp, you start by converting a regex into a state machine via the Compile or MustCompile methods. They ultimately do the same thing, but MustCompile will panic if the regex is invalid. You can then use the provided Regexp struct to find matches repeatedly. A Regexp struct is safe to use across goroutines.

re := regexp2.MustCompile(`Your pattern`, 0)
if isMatch, _ := re.MatchString(`Something to match`); isMatch {
    //do something

The only error that the *Match* methods should return is a Timeout if you set the re.MatchTimeout field. Any other error is a bug in the regexp2 package. If you need more details about capture groups in a match then use the FindStringMatch method, like so:

if m, _ := re.FindStringMatch(`Something to match`); m != nil {
    // the whole match is always group 0
    fmt.Printf("Group 0: %v\n", m.String())

    // you can get all the groups too
    gps := m.Groups()

    // a group can be captured multiple times, so each cap is separately addressable
    fmt.Printf("Group 1, first capture", gps[1].Captures[0].String())
    fmt.Printf("Group 1, second capture", gps[1].Captures[1].String())

Group 0 is embedded in the Match. Group 0 is an automatically-assigned group that encompasses the whole pattern. This means that m.String() is the same as m.Group.String() and m.Groups()[0].String()

The last capture is embedded in each group, so g.String() will return the same thing as g.Capture.String() and g.Captures[len(g.Captures)-1].String().

If you want to find multiple matches from a single input string you should use the FindNextMatch method. For example, to implement a function similar to regexp.FindAllString:

func regexp2FindAllString(re *regexp2.Regexp, s string) []string {
	var matches []string
	m, _ := re.FindStringMatch(s)
	for m != nil {
		matches = append(matches, m.String())
		m, _ = re.FindNextMatch(m)
	return matches

FindNextMatch is optmized so that it re-uses the underlying string/rune slice.

The internals of regexp2 always operate on []rune so Index and Length data in a Match always reference a position in runes rather than bytes (even if the input was given as a string). This is a dramatic difference between regexp and regexp2. It's advisable to use the provided String() methods to avoid having to work with indices.

Compare regexp and regexp2

Category regexp regexp2
Catastrophic backtracking possible no, constant execution time guarantees yes, if your pattern is at risk you can use the re.MatchTimeout field
Python-style capture groups (?P<name>re) yes no (yes in RE2 compat mode)
.NET-style capture groups (?<name>re) or (?'name're) no yes
comments (?#comment) no yes
branch numbering reset (?|a|b) no no
possessive match (?>re) no yes
positive lookahead (?=re) no yes
negative lookahead (?!re) no yes
positive lookbehind (?<=re) no yes
negative lookbehind (?<!re) no yes
back reference \1 no yes
named back reference \k'name' no yes
named ascii character class [[:foo:]] yes no (yes in RE2 compat mode)
conditionals (?(expr)yes|no) no yes

RE2 compatibility mode

The default behavior of regexp2 is to match the .NET regexp engine, however the RE2 option is provided to change the parsing to increase compatibility with RE2. Using the RE2 option when compiling a regexp will not take away any features, but will change the following behaviors:

  • add support for named ascii character classes (e.g. [[:foo:]])
  • add support for python-style capture groups (e.g. (P<name>re))
  • change singleline behavior for $ to only match end of string (like RE2) (see #24)
  • change the character classes \d \s and \w to match the same characters as RE2. NOTE: if you also use the ECMAScript option then this will change the \s character class to match ECMAScript instead of RE2. ECMAScript allows more whitespace characters in \s than RE2 (but still fewer than the the default behavior).
  • allow character escape sequences to have defaults. For example, by default \_ isn't a known character escape and will fail to compile, but in RE2 mode it will match the literal character _
re := regexp2.MustCompile(`Your RE2-compatible pattern`, regexp2.RE2)
if isMatch, _ := re.MatchString(`Something to match`); isMatch {
    //do something

This feature is a work in progress and I'm open to ideas for more things to put here (maybe more relaxed character escaping rules?).

ECMAScript compatibility mode

In this mode the engine provides compatibility with the regex engine described in the ECMAScript specification.

Additionally a Unicode mode is provided which allows parsing of \u{CodePoint} syntax that is only when both are provided.

Library features that I'm still working on

  • Regex split

Potential bugs

I've run a battery of tests against regexp2 from various sources and found the debug output matches the .NET engine, but .NET and Go handle strings very differently. I've attempted to handle these differences, but most of my testing deals with basic ASCII with a little bit of multi-byte Unicode. There's a chance that there are bugs in the string handling related to character sets with supplementary Unicode chars. Right-to-Left support is coded, but not well tested either.

Find a bug?

I'm open to new issues and pull requests with tests if you find something odd!


A full-featured regex engine in pure Go based on the .NET engine

License:MIT License


Language:Go 100.0%