Regular Expression Tester

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What this tool does

This tool tests a regular expression against a block of text using the browser's native JavaScript regex engine. Matches are highlighted directly in the text, every match is broken down into its numbered and named capture groups, and an optional replace mode shows exactly what String.replace() would produce with your pattern and replacement text.

How It Works

Your pattern and selected flags are compiled into a real JavaScript RegExp object. When the Global flag is on, the tool repeatedly calls exec() on the text, advancing past each match (and past empty matches by one character to avoid an infinite loop) until no more matches are found; without Global, exec() is called once. Highlighting is built by walking through the matches in order and wrapping each matched substring in a marker while leaving the surrounding text untouched. Replace mode calls the text's own replace() method with your compiled pattern and replacement string, so the result is exactly what your own JavaScript code would produce.

Problems it solves

• Debugging why a regex in your actual JavaScript code is not matching what you expect, against the exact same engine.
• Previewing a find-and-replace operation before running it on real data.
• Understanding which part of a pattern each capture group corresponds to, by seeing every group's captured text at once.
• Learning regex flags hands-on by toggling them and watching the match list change immediately.

Frequently asked questions