Regex Tester — Live Match Highlighting

How to use it

1. 1

   Type your pattern, without the surrounding slashes, and pick your flags.

2. 2

   Paste the text you want to test against.

3. 3

   Read the highlighted matches, and expand the match list to inspect capture groups.

4. 4

   Optionally enter a replacement string to preview the substitution.

Frequently asked questions

Which regex flavour is this?

JavaScript's built-in RegExp engine, running in your browser. Patterns behave exactly as they will in JavaScript or TypeScript, including named groups, lookbehind and unicode property escapes. Note that other flavours differ: PCRE, Python and Go's RE2 each have syntax this engine does not share, so verify elsewhere if your pattern is destined for one of those.

Is my test data sent anywhere?

No. Matching runs entirely in your browser with no request, so pasting production log lines or real records to test against is safe.

Why does my pattern match more than I expected?

Almost always a greedy quantifier. By default, star and plus consume as much as they can and then give back only as needed, so a pattern like a quoted-string matcher using dot-star will swallow everything up to the last quote on the line rather than the first. Add a question mark to make the quantifier lazy, or use a negated character class instead of dot.

What does the global flag change?

Without g, only the first match is found. With it, matching continues through the whole input, which is what you want when highlighting every occurrence or replacing all of them. This tool enables it by default so the highlighting shows everything.

My pattern hangs the page. What happened?

You have most likely hit catastrophic backtracking, where nested quantifiers create an exponential number of ways to fail a match. A classic shape is a group containing a quantifier that is itself quantified. This matters beyond the tool: the same pattern in production is a denial-of-service risk if it ever sees attacker-controlled input.