Regex101 Regular Expression Tester

Regex101 is a comprehensive online regular expression testing and debugging tool that supports PCRE2, JavaScript, Python, and Go regex flavors, enabling developers to build, test, and understand complex patterns with real-time matching feedback and detailed explanations. The interactive interface provides a split-pane layout where users enter their pattern in the top field, paste test strings in the bottom area, and receive immediate visual highlighting of all matches in the test string with color-coded capture groups and full match information. The explanation engine generates a human-readable breakdown of every token in the pattern, describing what each quantifier, character class, anchor, lookahead, lookbehind, and backreference does, making it invaluable for both learning regular expressions and debugging complex patterns. Advanced features include match information display showing full match text, capture group contents, and group positions, a quick reference panel listing common regex tokens and modifiers for each flavor, substitution capability to test replacement strings with capture group references, and unit test functionality to save and verify patterns against multiple test cases. The debugger mode steps through the matching process token by token, showing the engine state at each step including current position, backtracking events, and match attempts, which helps optimize patterns for performance and avoid catastrophic backtracking. Users can save and share patterns via unique URLs, enabling collaboration and documentation. The library section allows publishing useful patterns for the community to discover and reuse. Registered users can maintain a personal library of saved regex patterns with tags and descriptions. The tool processes millions of expressions monthly and is trusted by developers worldwide as the go-to resource for regular expression development.