On June 26, 2026, CircleCI is updating the regex engine used to evaluate when: matches: pattern: conditions in pipeline configs. This change improves the security and reliability of config compilation.
After this date, the following regex features will no longer be supported in pattern: fields and configs using them will fail to compile:
- Negative lookaheads
- Negative lookbehinds
- Possessive quantifiers
- Backreferences
Please refer to our Discuss post for full details with regex examples and fixes.