Regex Tester — Test Regular Expressions in Browser, No Upload
Test JavaScript regular expressions against any input with live match highlighting. Supports i, m, and s flags. Runs entirely in your browser — no upload, no server, no logs.
How to use this tool
- Enter your regular expression in the pattern field.
- Paste the text to test against.
- Review highlighted matches and captured groups.
About Regex Tester
A regular expression is a compact pattern language for describing sets of strings, used to search, validate, extract, and replace text. A pattern is built from literal characters plus metacharacters that carry special meaning: the dot matches any character, character classes in square brackets match one of a set, quantifiers like *, +, ?, and {n,m} control repetition, and anchors like ^ and $ tie a match to the start or end of the input. Parentheses create capturing groups, so you can pull specific pieces out of a match rather than just confirming the whole thing matched.
This tester uses the JavaScript RegExp engine — the same one that runs in Node.js and every modern browser — so the behavior you see here matches what your application code will do. It exposes the i, m, and s flags. The i flag makes matching case-insensitive. The m (multiline) flag changes ^ and $ so they match at the start and end of each line rather than only the whole string. The s (dotall) flag lets the dot also match newline characters, which it normally does not. Internally the global flag is always applied so that every non-overlapping match in your input is found and highlighted, not just the first.
Capturing groups are reported alongside each match, including named groups written as (?<name>...), which makes it easy to confirm you are extracting the right substrings before wiring the pattern into code. A subtle point the tester handles for you is zero-length matches: a pattern that can match the empty string would otherwise loop forever during a global scan, so the engine advances past empty matches to keep results correct. If your pattern is syntactically invalid — an unbalanced bracket or a bad quantifier — you get the engine's error message rather than silent failure.
Two practical cautions. First, regex flavors differ: JavaScript supports lookahead and, in modern engines, lookbehind, but it does not support some features found in PCRE or Python such as recursive patterns or possessive quantifiers, so a pattern copied from another language may behave differently. Second, certain patterns with nested quantifiers can trigger catastrophic backtracking, where matching a non-matching string takes exponential time and can hang a page — test against realistic worst-case input. Regex is also the wrong tool for parsing genuinely nested structures like arbitrary HTML or JSON; use a real parser there. All matching here happens in your browser, so your test input is never sent anywhere.
Frequently Asked Questions
- Which regex flavor does this tester use?
- It uses the native JavaScript RegExp engine, the same implementation found in Node.js and modern browsers. That means the matching behavior here is identical to what your JavaScript or TypeScript code will produce, so patterns you verify can be pasted straight into your project.
- What do the i, m, and s flags do?
- The i flag makes matching case-insensitive. The m (multiline) flag makes the ^ and $ anchors match at the start and end of each line instead of only the whole input. The s (dotall) flag allows the dot to match newline characters, which it otherwise skips. You can toggle any combination of them.
- Does it find every match or just the first?
- Every match. The tester always applies the global flag internally, so it returns all non-overlapping matches across your input and highlights each one. This is why you do not need to add g yourself to see complete results.
- Can I use capturing groups, including named ones?
- Yes. Both numbered groups created with parentheses and named groups written as (?<name>...) are supported, and the captured values are shown for each match. This lets you confirm you are extracting exactly the substrings you intend before relying on the pattern in code.
- Why does a pattern from Python or PCRE behave differently here?
- Regex flavors are not identical. JavaScript lacks some features available in other engines, such as recursive subpatterns and possessive quantifiers, and certain escape sequences differ. A pattern written for another language may need adjustment, so testing it against the JavaScript engine you will actually run is the safest approach.
- What is catastrophic backtracking and should I worry about it?
- It is a performance trap where a pattern with nested or overlapping quantifiers takes exponentially long to fail on certain inputs, which can freeze the page or a server. If your pattern processes untrusted input, test it against long non-matching strings and simplify ambiguous quantifiers to avoid the worst case.
- Can I use regex to parse HTML or JSON?
- It is a poor fit for genuinely nested or recursive structures like arbitrary HTML or JSON, because regular expressions cannot reliably track nesting. Use a dedicated parser for those. Regex is excellent for flat, well-defined patterns such as validating an email shape, extracting dates, or finding tokens.
- Is my test input sent to a server?
- No. The pattern and the text are matched entirely in your browser. Nothing is uploaded or logged, so you can test against real or sensitive sample data without it leaving your device.
Embed this tool on your site
Free. One line of HTML — the tool runs in your visitor's browser, no data sent to anyone.
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