Regex Tester & Explainer
Test a regular expression against your own text, see every match highlighted as you type, and read back in plain English what the pattern actually does. Nothing leaves your browser.
Frequently asked questions
Which regex flavour does this test?
JavaScript’s own RegExp engine, exactly as it behaves in a browser and in Node.js. Patterns that rely on PCRE, Python or .NET extensions — possessive quantifiers, atomic groups, recursion, K — are not supported here because they are not supported in JavaScript either.
What do the g, i, m, s, u and y flags do?
g finds every match rather than just the first, i ignores case, m makes ^ and $ match at each line rather than only at the ends of the text, s lets . match a newline, u turns on full Unicode handling and y anchors matching at the current position. Each flag is explained in place as you toggle it.
Why does my pattern only match once?
Because the g flag is off. Without it the engine stops at the first match — which is also what String.replace and RegExp.exec do — so a pattern that looks correct appears to miss later occurrences. The tool points this out and offers to add the flag for you.
How do I test a replacement?
Switch to the replace view and type the replacement string. Group references such as $1 and named references such as $<year> are substituted as String.replace would substitute them, so what you see is what your code will produce.
Why did matching stop partway through?
A pattern that backtracks catastrophically — typically nested quantifiers like (a+)+ over a long non-matching string — can run effectively forever. Matching is abandoned after 250 milliseconds so the page stays usable, and the warning is a real finding: the same pattern would hang your server, which is how regular-expression denial of service happens.
What does the explanation panel do?
It reads the pattern apart and describes each piece in plain English — what a character class covers, what a quantifier repeats, what a group captures — nested to match the structure. It is the fastest way to work out what an inherited pattern actually does before you change it.
Is the text I test against sent anywhere?
No. The whole tool is JavaScript running in your own browser, so nothing you type or paste is sent over the network, written to a log or attached to an analytics event. You can confirm it by opening the network tab and watching it stay empty while you work.