What causes "mux: error compiling regex for %q: %w" in gorilla/mux?

Fix the pattern syntax (balance brackets; use (?:...) for groups).

mux: error compiling regex for %q: %w

Error message

mux: error compiling regex for %q: %w

What it means

Returned by newRouteRegexp (regexp.go:109-111) wrapping the underlying error when the per-variable pattern in {name:pattern} fails to compile via RegexpCompileFunc (regexp.Compile by default). The %w lets callers errors.Unwrap to the regexp error. Stored on r.err during route construction.

Source

Thrown at regexp.go:111

		}

		// Name or pattern can't be empty.
		if name == "" || patt == "" {
			return nil, fmt.Errorf("mux: missing name or pattern in %q", tag)
		}
		// Build the regexp pattern.
		groupName := varGroupName(groupIdx)

		pattern.WriteString(regexp.QuoteMeta(raw) + "(?P<" + groupName + ">" + patt + ")")

		// Build the reverse template.
		reverse.WriteString(raw + "%s")

		// Append variable name and compiled pattern.
		varsN[groupIdx] = name
		varsR[groupIdx], err = RegexpCompileFunc("^" + patt + "$")
		if err != nil {
			return nil, fmt.Errorf("mux: error compiling regex for %q: %w", tag, err)
		}
	}
	// Add the remaining.
	raw := tpl[end:]
	pattern.WriteString(regexp.QuoteMeta(raw))
	if options.strictSlash {
		pattern.WriteString("[/]?")
	}
	if typ == regexpTypeQuery {
		// Add the default pattern if the query value is empty
		if queryVal := strings.SplitN(template, "=", 2)[1]; queryVal == "" {
			pattern.WriteString(defaultPattern)
		}
	}
	if typ != regexpTypePrefix {
		pattern.WriteByte('$')
	}

View on GitHub (pinned to db9d1d0073)

Solutions

Fix the pattern syntax (balance brackets; use (?:...) for groups).
Test-compile patterns offline with regexp.Compile before registering.
If you override RegexpCompileFunc, set it once at init before any route is built (not safe for concurrent use).

Example fix

// before
r.Path("/u/{id:[0-9}")
// -> mux: error compiling regex for "{id:[0-9}": ...

// after
r.Path("/u/{id:[0-9]+}")

Defensive patterns

Strategy: try-catch

Validate before calling

func compileCheck(p string) error {
    _, err := regexp.Compile(p)
    return err
}

Type guard

func validPattern(p string) bool { _, err := regexp.Compile(p); return err == nil }

Try / catch

rt := r.Path(tpl)
if err := rt.GetError(); err != nil {
    return fmt.Errorf("route %q invalid: %w", tpl, err)
}

Prevention

Test-compile every variable pattern at startup.
Only set RegexpCompileFunc at init, never concurrently.
Prefer simple character classes over complex RE2.

When it happens

Trigger: r.Path("/u/{id:[0-9}") (unbalanced bracket); r.Path("/u/{s:a(b}"); r.Host("{x:??}"). Any invalid RE2 syntax in a variable pattern.

Common situations: Typing a pattern with unbalanced brackets/parens; using PCRE features RE2 rejects (backreferences, lookahead); overriding RegexpCompileFunc with a stricter engine after routes are built.

Related errors

AI-assisted analysis of gorilla/mux@db9d1d0073 (2026-08-04). Data as JSON: /data/errors/53d1523a978b0868.json. Report an issue: GitHub.