ToolBirch

Regex Tester

Test JavaScript regular expressions with live match highlighting.Developer Tools

Regex Tester

Test a JavaScript regular expression against sample text and see every match highlighted.

About this tool

Dial in a pattern before it goes into production code. The tester uses the browser's own RegExp engine, so lookaheads, named groups and Unicode classes behave exactly as they will at runtime.

How to use the Regex Tester tool

  1. Write the pattern. Enter the expression and any flags such as g, i or m.
  2. Paste sample text. The text the pattern should run against.
  3. Read the matches. Every match is highlighted inline and counted; compile errors are reported verbatim.

Supported formats

Input

  • A JavaScript RegExp pattern
  • Flag characters (g, i, m, s, u, y)

Output

  • Highlighted matches
  • Match count
  • Compile error messages

Inputs are validated before anything is processed. Invalid values and unsupported files are rejected with a clear error instead of failing silently.

Frequently asked questions

Which regex dialect?

The browser's JavaScript engine — PCRE-only features such as K or recursive patterns are not available.

Are capture groups shown?

Matches are highlighted; the group breakdown is not listed in this first release.

Is there a match limit?

Iteration stops at 500 matches to keep the page responsive; a real count above that is not shown.

Why does my pattern error?

The message comes straight from RegExp, so an unbalanced parenthesis or bad escape is reported as the engine sees it.

Privacy

Everything you enter stays in this browser tab. There is no upload endpoint for tool inputs, no account, and no server-side copy of your text or files. Results are produced locally and disappear when you close the page.

No account is required and no file is ever uploaded. See the privacy policy for details about analytics and the limited data this site stores.

Known limitations

  • Match iteration is capped at 500 for responsiveness.
  • Capture group tables and replace-mode previews are not offered.
  • Catastrophic backtracking can still freeze the tab, exactly as it would in production.

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