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Regex Tester

Test JavaScript regex patterns with highlighted matches, capture groups, named groups and a replacement preview. Runs in a safe worker in your browser.

Flags

Enter a pattern to see matches.

Processed locally in your browser. Your data never leaves your device.

About this regex tester

A regular expression describes a pattern of text. This tester runs JavaScript regular expressions against your text as you type, highlights every match, lists the capture groups (numbered and named), and previews the result of a replacement. It runs in a background worker with a time limit, so even a badly written pattern cannot freeze the page.

How to use it

  1. Type a pattern between the slashes and choose flags.
  2. Paste the text to test. Matches are highlighted in alternating colours.
  3. Read the match list for positions and capture groups.
  4. Optionally type a replacement to preview the result, and copy it.

Quick reference

  • \d digit, \w word character, \s whitespace, . any character except a line break.
  • * zero or more, + one or more, ? optional, {2,4} between 2 and 4. Add ? for the lazy version.
  • ^ start, $ end, \b word boundary.
  • (…) capture group, (?<name>…) named group, (?:…) non-capturing, (?=…) and (?<=…) lookahead and lookbehind.

Avoiding slow patterns

  • Avoid nested quantifiers such as (a+)+ or (.*)*: they can take exponential time when the match fails.
  • Be specific. [^,]* is safer than .* when you mean "up to the next comma".
  • Anchor the pattern when you can, and never run untrusted patterns on your own server without a time limit.

Frequently asked questions

Which regex flavour does this use?
JavaScript (ECMAScript), the same engine your browser uses. Patterns from PCRE, Python or .NET may differ: for example there are no possessive quantifiers or atomic groups, and lookbehind and named groups use the JavaScript syntax.
Why did my pattern stop after 2 seconds?
Some patterns take exponential time on certain inputs, known as catastrophic backtracking. A classic example is (a+)+$ against a long run of a followed by b. Each run happens in a background worker that is terminated after 2 seconds, so the page never freezes. Remove the nested quantifiers or make the pattern more specific.
What do the flags do?
g finds all matches, i ignores case, m makes ^ and $ match at every line, s lets . match line breaks, u and v turn on Unicode modes (v is stricter and cannot be combined with u), y matches only at the current position, and d records match positions, which this tool always uses.
How do I reference groups in the replacement?
Use $1, $2 for numbered groups, $<name> for named groups, $& for the whole match and $$ for a literal dollar sign.
Is my text sent to a server?
No. Patterns run in a worker in your browser, and neither your pattern nor your text is uploaded, stored or logged.