Regex Tester

Q: Why did my pattern get stopped?

Some patterns trigger catastrophic backtracking and could freeze the page. This tool runs matching in a background thread and stops any pattern that takes longer than half a second.

About this tool

This free online regex tester highlights every match in real time as you edit the pattern, flags, or test string. Your text is never uploaded: matching runs locally in your browser, so it is safe to paste logs, source code, or any data you would not send to a random server. It uses the JavaScript regex flavor — the same engine that runs in Node.js and every browser.

How to use

Pattern — type a regular expression between the slashes; syntax errors are reported immediately below.
Flags — toggle g, i, m, s, u, and y to change how matching behaves.
Test string — paste or type text in the editor; matches are highlighted in alternating colors so adjacent matches stay distinguishable.
Match list — every match is listed with its character range and capture groups (numbered and named); click one to scroll the editor to it.
Replace — open the Replace row and enter a replacement pattern to preview the substituted text live. $1 inserts capture group 1, $<name> a named group, $& the whole match, and $$ a literal dollar sign.

Flag reference

g global — find all matches instead of stopping at the first one.
i ignoreCase — match letters regardless of case.
m multiline — make ^ and $ match at the start and end of each line, not just the whole string.
s dotAll — let . match newline characters too.
u unicode — treat the pattern as Unicode; surrogate pairs (like emoji) count as one character and \p{...} property escapes become available.
y sticky — only match starting exactly at the current position, useful when writing tokenizers.

FAQ

Is my text uploaded to a server? No. Matching runs in a Web Worker inside your browser. Nothing leaves your machine.

Which regex flavor does this use? JavaScript (ECMAScript). It supports lookahead, lookbehind, named capture groups (?<name>...), and Unicode property escapes, but not PCRE-only features such as recursion or possessive quantifiers.

Why did my pattern get stopped? Some patterns (like (a+)+$ against a long string of a's) trigger catastrophic backtracking and could freeze the page. This tool runs matching in a background thread and stops any pattern that takes longer than half a second.