Regex Tester

JavaScript’s RegExp engine, so what you test here behaves identically in Node and the browser. Most syntax carries over to other languages, but be careful with lookbehind, named groups and Unicode property escapes — support for those varies between PCRE, Python and Go, so verify before porting a pattern.

g finds every match rather than stopping at the first. i ignores case. m makes ^ and $ match at each line rather than only the whole string. s lets the dot match newlines. u enables full Unicode handling. Most everyday patterns want g and i; add m when working line by line.

Almost certainly catastrophic backtracking — nested quantifiers like (a+)+ force the engine to try an exponential number of paths on a non-matching input. It is a real denial-of-service risk if the pattern runs on user input. Make the inner parts more specific, or replace greedy quantifiers with lazy or possessive alternatives.

Yes, Regex Tester is completely free. No signup, no account, and no watermark on outputs. A Pro tier is available for 100 AI ops/day and larger file sizes.

Regex Tester runs entirely in your browser. Your file is never uploaded — it is read, processed and saved locally, so it never reaches our servers or anyone else's. You can disconnect from the internet after the page loads and it still works.