Regex Tester

JavaScript (ECMAScript) regular expressions — the same engine your browser and Node.js use. It differs from PCRE or Python's re module in some edge cases: named group syntax is (?<name>...) in all three, but lookbehind support ((?<=...) and (?<!...)) varies by browser, and some escape sequences behave slightly differently. If you're testing a pattern meant for another language, treat this as close, not identical.

g (global) finds all matches instead of stopping at the first. i (case insensitive) ignores letter case. m (multiline) makes ^ and $ match the start/end of each line rather than the whole string. s (dotAll) lets . match newline characters too, which it normally doesn't. u (unicode) treats the pattern as Unicode code points instead of raw UTF-16 units, which matters for characters outside the Basic Multilingual Plane like many emoji.

Certain patterns — typically ones with nested repetition, like (a+)+ — can cause "catastrophic backtracking": the engine tries an exponential number of ways to match before giving up, and a moderately long input can make that take longer than the age of the universe. This tool runs every evaluation in a background worker with a hard time limit, so a runaway pattern gets stopped and flagged rather than freezing your browser tab.

No. Pattern matching and replacement happen entirely in your browser, in a background worker. Nothing is uploaded, logged, or sent anywhere.