Developer Tools
Regex Tester
Including the zero-length match trap that makes a pattern loop for ever.
- Runs in your browser
- Nothing uploaded
The tool
Enter a pattern, any flags, and the text to test it against. Matches appear as you type.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Working out why a pattern matches one string and not another
- Checking a validation rule before putting it in production
- Extracting fields from a log line
- Testing a redirect or rewrite pattern
- Learning what a pattern somebody else wrote actually does
About this tool
JavaScript flavour
This uses the browser own regular expression engine, so what you see is exactly what JavaScript will do. That matters because flavours differ: lookbehind, named groups and Unicode property escapes are supported in modern browsers but not everywhere, and PCRE, Python and Go each have their own quirks. A pattern proven here is proven for JavaScript, not universally.
Flags
The global flag is added automatically, because without it you only ever see the first match. Add i for case insensitivity, m to make ^ and $ match at line breaks, s to let the dot match newlines, and u for proper Unicode handling.
The zero-length match trap
A pattern that can match nothing, such as a*, will match an empty string at every position. With the global flag, a naive loop never advances and hangs the page. This tool nudges the index forward when that happens, which is the same guard your own code needs if it loops over matches manually.
Reading the groups
Every capturing group is listed under its match. A group that did not participate shows as no match rather than an empty string, which is a distinction that matters: an optional group that matched nothing and one that matched an empty string behave differently in most languages.
Need this handled across a whole website? See our Backend Web Development and Custom Web Apps page.
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If you would rather someone did this properly for you, we offer Web Design Services and Website Development, SEO Services: Search Engine Optimization, AI Automation Agency.
Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
JavaScript, because it uses the browser own engine. Patterns may behave differently in PCRE, Python or Go.
Without it you only ever see the first match, which is rarely what you want from a tester.
A cap, so a pattern matching thousands of times cannot freeze the page.
That the group did not participate in the match at all, which is different from matching an empty string.
No. The matching happens in your browser.
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