Regex tester
Test regular expressions with real-time match highlighting, flag controls, and a match table. Client-side only.
Client-side only — Uses the browser's native RegExp engine. Capped at 500 matches to prevent ReDoS. Zero-width matches advance the cursor automatically to prevent infinite loops.
Guide
Regex Tester: what it does and how to use it
What this Developer tool does
This regex tester helps you write and debug regular expressions with live matches. It is useful for log parsing, validation rules, search patterns, capture groups, and quick experiments before adding a pattern to code.
How it works
Enter a pattern, flags, and sample text. The browser evaluates matches locally and highlights results with capture information. Because regex behavior can differ between engines, test final patterns in the runtime where they will be used.
Examples
- Extract IDs from log lines.
- Test email-like validation patterns.
- Debug capture groups before writing code.
Security considerations
Inputs stay in the browser. Avoid running untrusted catastrophic patterns against very large text in production systems.
FAQ
Which regex engine is used?
The browser JavaScript RegExp engine.
Can regex validate all emails?
Not perfectly. Use practical validation and server-side confirmation for real accounts.
What are flags?
Flags such as g, i, and m change matching behavior like global search, case-insensitive matching, and multiline mode.