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Regex 17 errors

Regular Expression Errors

Catastrophic backtracking, back references, escaping and engine differences.

Understanding 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.

How to debug Regex errors

  1. 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.
  2. Eliminate nested quantifiers over overlapping classes. Rewrite (a+)+ as a+, and prefer possessive quantifiers or atomic groups where the engine supports them.
  3. 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.
  4. Anchor patterns with ^ and $ where you mean a whole-string match. Unanchored patterns silently match substrings.
  5. 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

All 17 Regex errors

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