Regex Tester
Test a regular expression against your text: every match, its position and its groups.
Runs entirely in your browser
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How to use
- Type a pattern — no surrounding slashes needed.
- Turn on the flags you want; g is applied for the preview so you see every match.
- Paste the text to search. Matches are highlighted as you type.
- Add a replacement to preview a substitution, using $1 or $<name> for groups.
Features
- Every match with its position, numbered groups and named groups.
- Highlighting that reassembles to exactly your original text.
- Replacement preview with $1 and $<name> references.
- Warns about patterns that can hang a browser, and about ones that match nothing.
- Runs in your browser — nothing you paste is uploaded.
Frequently asked questions
- Why does it show every match when I have not set the g flag?
- Because a tester that showed one match for a pattern with five would be describing itself rather than your pattern. The preview always searches globally; the warning reminds you that in code, without g, you get only the first match. Replacement respects the flag exactly — with g it replaces everywhere, without it once.
- What is catastrophic backtracking?
- Some patterns take exponential time on inputs that nearly match. `(a+)+b` against a long run of a's tries every way of splitting them before giving up. JavaScript's engine cannot be interrupted once matching starts — there is no timeout — so a browser tester can only limit what it feeds in. This one caps the subject and the match count, and warns on the shapes that cause it. That is a mitigation, not a guarantee.
- Why is there a length limit on the text?
- Twenty thousand characters, for the reason above: the risk of a hung tab grows with the subject, and the tab cannot be rescued once a bad match begins. Long enough for real text, short enough to stay survivable.
- Is this the same regex my language uses?
- It is JavaScript's, which differs from PCRE, Python and Go in places — lookbehind support, named group syntax, and how unicode properties work. Patterns that use only the common syntax behave the same everywhere; anything exotic is worth checking in the language you will ship it in.
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