How to Debug a Regular Expression
Regex debugging becomes easier when you separate the pattern, flags, test cases, and expected matches. This guide focuses on JavaScript regular expressions.
Start with a minimal test
Use the smallest input that demonstrates the problem. Large real-world text can hide which part of the pattern is responsible.
Check flags
Global, case-insensitive, multiline, and Unicode-related behavior can change the result. Make sure the flags match the behavior your application expects.
Escape special characters
Characters such as `.`, `+`, `*`, `?`, `(`, `)`, `[`, `]`, `^`, and `$` have special meanings in regex syntax. Escape them when you need their literal form.
Test negative cases
A useful validation regex needs tests that should match and tests that must not match. The Regex Tester can help you iterate before putting the expression into code.