Regex: unbalanced parenthesis / missing closing bracket
The pattern has an unmatched grouping construct. In dynamically built patterns this is nearly always user input interpolated without escaping, which is also an injection risk.
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.
# Always escape interpolated literals
// JavaScript
const esc = s => s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
new RegExp(`^${esc(userInput)}$`);
# Python
re.compile(re.escape(user_input))
# Go
regexp.QuoteMeta(userInput)
# Debug a complex pattern by building it in named pieces with the
# extended/verbose flag: Python re.VERBOSE, PCRE (?x).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+)+asa+, 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.comre2 / RE2JPython re.DEBUGgrep -P for PCRE testingrecheck / redos linters
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Regex errors
- Python: re.error: bad escape \d at position 0The replacement argument of re.sub is not a pattern. It understands only group references…
- Regex: Catastrophic backtracking (ReDoS)A regular expression takes exponential time on certain inputs due to nested quantifiers or…
- Regex: Invalid back referenceThe regex references a capture group that doesn't exist (e.g., \3 when there are only 2…
- Regex: Invalid regular expression - unexpected characterA special regex metacharacter (. * + ? | ^ $ [ ] { } ( ) \) was used without escaping…
- Regex: Lookbehind not supportedThe regex engine does not support lookbehind assertions (or only supports fixed-width…
- Regex: nothing to repeat at position 0A quantifier appeared with nothing before it to repeat, so the pattern is invalid rather than…
- Regex: test() returns false every other call with the /g flagA regex with the g or y flag keeps lastIndex between calls, so test() resumes from where the…
Browse other categories
- HTTP 494xx client errors, 5xx server errors, redirects, headers and protocol problems.
- JavaScript 42npm resolution, async pitfalls, hydration, memory limits and runtime type…
- Database 41Connections, deadlocks, constraints, replication and memory limits.
- AI 35Rate limits, context windows, GPU memory and model-serving failures.
- Network 35Refused connections, timeouts, resets, MTU problems and port exhaustion.
- Python 35Imports, virtual environments, encoding, concurrency and dependency conflicts.
- Kubernetes 34CrashLoopBackOff, ImagePullBackOff, OOMKilled, RBAC, scheduling and storage.
- Docker 27Daemon connectivity, disk space, image pulls, ports and architecture mismatches.
- System 26Disk space, systemd units, file descriptors, OOM killer and scheduled jobs.
- Cloud 25IAM permissions, quotas, service limits and credential failures.
- Security 25JWT validation, CSRF, OAuth grants, SELinux, SSH host keys and CSP.
- TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.
Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.