Regex: Catastrophic backtracking (ReDoS)
A regular expression takes exponential time on certain inputs due to nested quantifiers or overlapping alternations. Can freeze applications or be exploited as a denial-of-service.
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.
# Avoid nested quantifiers
# BAD: (a+)+$
# GOOD: a+$
# Use atomic groups or possessive quantifiers
# Use specific character classes instead of .
# Test with regex debugger tools
# Use RE2 or rust regex for linear-time matchingHow 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: 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…
- Regex: unbalanced parenthesis / missing closing bracketThe pattern has an unmatched grouping construct. In dynamically built patterns this is nearly…
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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.