Regex New Added 19 September 2026

Go: error parsing regexp: invalid or unsupported Perl syntax: (?!

Go uses RE2, which has no backtracking and therefore no lookahead, no lookbehind and no backreferences. That is a deliberate guarantee of linear time matching rather than a missing feature, so the pattern has to be restructured instead of escaped.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Rejected at compile time, and MustCompile panics at init
regexp.MustCompile(`foo(?!bar)`)   // negative lookahead
regexp.MustCompile(`(\w)\1`)       // backreference

// Match the wider thing, then filter in Go
re := regexp.MustCompile(`foo(\w*)`)
for _, m := range re.FindAllStringSubmatch(s, -1) {
    if !strings.HasPrefix(m[1], "bar") {
        use(m[0])
    }
}

// Negated character classes cover many "not followed by" cases directly
regexp.MustCompile(`foo[^b]`)

// Compile at package level so a bad pattern fails on start, not per request
var idRe = regexp.MustCompile(`^[a-z0-9-]{1,64}$`)

// If the pattern comes from config, report the error rather than panicking
re, err := regexp.Compile(pattern)
if err != nil { return fmt.Errorf("bad pattern %q: %w", pattern, err) }

How to diagnose Regex errors

Regex problems come in two flavours: it does not match what you expect (usually escaping, greediness, or an engine feature difference) and it matches but takes forever. The second is catastrophic backtracking, a genuine denial-of-service vector known as ReDoS, caused by nested quantifiers over overlapping character classes, such as (a+)+b. Any regex applied to untrusted input should be checked for it.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Test against a visualiser that shows backtracking steps, and always test with input that fails to match. That is where catastrophic backtracking appears, not on successful matches.
Eliminate nested quantifiers over overlapping classes. Rewrite (a+)+ as a+, and prefer possessive quantifiers or atomic groups where the engine supports them.
Know your engine: lookbehind, named groups and Unicode property escapes differ between PCRE, RE2, JavaScript, Python and Go. RE2 (used by Go) deliberately has no backtracking and rejects some patterns outright.
Anchor patterns with ^ and $ where you mean a whole-string match. Unanchored patterns silently match substrings.
Set a timeout or use a linear-time engine for user-supplied patterns. Never run an untrusted regex on a request thread.

Tools worth reaching for

regex101.com
re2 / RE2J
Python re.DEBUG
grep -P for PCRE testing
recheck / redos linters

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

pkg.go.dev pkg.go.dev

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