Matches are highlighted here live. Your text never leaves this page.
Replace
A live preview of .replace() appears here once you have a valid pattern and some test text.
Frequently asked questions
Which regex flavor does this tester use?
It uses JavaScript's built-in RegExp engine, exactly as it behaves in browsers and Node.js. That includes named capture groups, lookbehind, the s and u flags, and $<name> replacements. Patterns may differ slightly from PCRE or Python's re, so always test in the flavor your code actually runs.
Why does my pattern hang or run slowly?
Nested quantifiers like (a+)+ can trigger catastrophic backtracking, where the engine tries exponentially many paths before failing. Prefer specific character classes over .*, anchor the pattern, and avoid overlapping quantifiers. This tester caps output at 5000 matches and catches errors, so a bad pattern degrades gracefully rather than crashing.
Is my test text sent anywhere?
No. The pattern, flags, test string, and replacement all stay in your browser; matching runs locally with JavaScript and nothing is transmitted, stored, or logged. You can safely paste logs, config files, or production data that you would not want leaving your machine while debugging an expression.