A regex that compiles without error and a regex that matches what you actually meant are two very different things, and the gap between them is where most bugs live.
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Regex Tester & Explainer
Live match highlighting, capture groups, a replace preview, and a plain-English breakdown of your pattern. Nothing you type is sent to a server.
How the explanation is generated
The pattern is parsed left to right into tokens, character classes, quantifiers, groups, anchors, escape sequences, and literal characters, each one described in plain English as it appears. It's a structural description of what you wrote, not a guess at what you meant, which is exactly what makes it useful for catching a pattern that parses correctly but doesn't do what you intended.
Regex tester FAQ
What regex flavor does this use?
JavaScript's native regex engine (the same one behind every "g"/"i"/"m" flag you'd use in a browser or Node.js). It's very close to PCRE used by PHP and Python for common patterns, but named groups, lookbehind support, and a few edge cases differ between languages, always test against your actual runtime for anything critical.
Why does my pattern explanation look different from another regex tool?
The explanation is generated by parsing your pattern token by token, character classes, quantifiers, groups, anchors, and describing each one in plain English as it appears left to right. It describes structure, not intent, so a technically valid but oddly-written pattern will get an accurate but possibly odd-sounding explanation.
What do the flags (g, i, m, s, u) actually change?
g (global) finds every match instead of stopping at the first. i (case-insensitive) ignores case. m (multiline) makes ^ and $ match the start/end of each line instead of just the whole string. s (dotall) lets . match newline characters too, which it normally doesn't. u (unicode) treats the pattern as a sequence of Unicode code points, important for emoji and non-Latin scripts.
Can I test a replace pattern here, not just matching?
Yes. Switch to replace mode, enter a replacement string using $1, $2 (or $<name> for named groups) to reference captured groups, and the result updates live against your test string.
Is my pattern or test text uploaded anywhere?
No. Everything, matching, highlighting, and the explanation, runs in your browser using JavaScript's built-in RegExp engine. Nothing is sent to a server.
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Why a pattern explanation beats trial and error
The usual way to debug a regex is to keep throwing test strings at it until it either works or you give up and rebuild it from scratch. The breakdown below this tool takes a different approach: it reads your pattern left to right and describes, in plain English, exactly what each piece does, a quantifier folded onto the token before it, a character class spelled out, a lookahead labeled as a lookahead. That turns “why doesn’t this match” into “oh, I quantified the wrong group,” which is usually a much faster fix.
The seven patterns people actually search for
Email, URL, IPv4 address, hex color, date, phone number, and postcode validation account for a huge share of every regex written for a web form. The presets above aren’t decorative, they’re working patterns you can drop straight into your own code, each paired with a sample string so you can see immediately what they do and don’t catch. The UK postcode pattern in particular trips people up more than it should: postcodes range from four to seven characters with an irregular structure (compare “M1 1AE” to “SW1A 1AA”), which is exactly the kind of edge case worth testing before you ship a signup form that silently rejects real customers.
Why matching behavior can differ between JavaScript and your backend
This tool runs on JavaScript’s native regex engine, the same one in every browser and in Node.js. Most everyday patterns behave identically in PHP, Python, and Java too, but named capture groups, lookbehind assertions, and a handful of edge cases in how Unicode is handled genuinely differ between engines. If a pattern is going into a backend validation rule rather than client-side JavaScript, test it again in that actual language before trusting it in production.
Replace mode, for more than just testing
Switching to replace mode runs your pattern through String.replace() with your replacement text, backreferences and all, so you can see the transformed result before you commit the same line to a script that processes a much larger file. It’s a fast way to sanity-check a find-and-replace pattern, reformatting a list of dates or normalizing whitespace, without risking it on real data first.