Golang Regex Tester

• Go uses RE2: no lookahead, no lookbehind, no backreferences — they won't compile
• The trade: every Go regex is guaranteed linear-time — ReDoS is impossible by design
• Named groups are (?P<name>…); read them via FindStringSubmatch + SubexpNames
• Flags are inline: (?i) case-insensitive, (?s) dot-matches-newline, (?m) multiline
• Copyable Go snippet with regexp.MustCompile and FindAllString included

Frequently Asked Questions

Why doesn't my lookahead work in Go?

Go's regexp package implements RE2, which does not support lookahead, lookbehind, or backreferences — they cause a compile error, not a silent failure. This is by design: RE2 guarantees linear-time matching. Rewrite trailing-context lookaheads as capture groups, and multi-condition lookaheads (password rules) as separate MatchString checks combined in Go code.

Can a Go regex be exploited with ReDoS?

No. RE2's linear-time guarantee means patterns like (a+)+b that hang backtracking engines run in linear time or fail to compile in Go. If you accept user-supplied patterns, Go is the safest mainstream language to do it in — though you should still cap pattern length and use regexp.Compile (not MustCompile) to handle invalid patterns without panicking.

How do I get named capture groups out in Go?

Define them as (?P<name>…), then pair FindStringSubmatch (values) with SubexpNames (names): loop over SubexpNames and index into the submatch slice. There's no map-style access built in — a small helper that zips the two slices is the standard idiom.

What's the Go equivalent of the g flag?

The FindAll* family: FindAllString(s, -1) returns every match (the -1 means no limit), FindAllStringSubmatch adds capture groups, and ReplaceAllString always replaces every occurrence. The generated snippet above uses FindAllString.

Is this tester free and private?

Yes — free, no signup, no limits, and 100% client-side: patterns and test strings are processed in your browser and never sent to a server.