About Regex Tester

Test regex patterns with match highlighting and group extraction. Part of the DevTools Surf developer suite. Browse more tools in the Text / String collection.

Use Cases

• Build and debug validation patterns for forms or inputs
• Extract structured data from log files or CSVs
• Verify search-and-replace rules before running them in production
• Test lookahead and lookbehind assertions interactively

Tips

• Flags like /g, /i, /m are supported
• Named groups (?<name>...) are highlighted separately
• Match count and group extractions shown in results

Fun Facts

• Regular expressions were invented by mathematician Stephen Kleene in the 1950s. The term 'regex' wasn't coined until decades later.
• Some regex patterns can take exponential time to evaluate — a phenomenon called 'catastrophic backtracking' that has caused real-world outages at Cloudflare and Stack Overflow.
• Perl's regex engine was so influential that most modern regex flavors are called 'Perl-compatible' (PCRE), even in languages that have nothing else to do with Perl.

FAQ

Which flavor does the tester use?

JavaScript's RegExp engine — the same your browser runs. For server-side flavors (PCRE, Python, Go), the results are usually identical for common patterns.

Does it show matched groups?

Yes — every capture group (numbered and named) is extracted for each match. Overlapping matches aren't produced — use a global regex with `g` flag for all matches.

Is there a performance warning?

Yes — the tester detects patterns prone to catastrophic backtracking (nested quantifiers) and warns before running. Prevents freezing the browser on adversarial input.

Can I use flags?

All JS flags: g, i, m, s, u, y. Hover each flag to see its effect. The tester re-runs as you toggle.