Which regex flavor does this use?

JavaScript's native RegExp — the same engine that runs in your browser and Node.js. Patterns that work here will work in your JS code.

What does each flag mean?

g = global (all matches), i = case-insensitive, m = multiline (^/$ match line boundaries), s = dotall (. matches newlines), u = unicode, y = sticky.

With the g flag, RegExp.exec() and .match() advance lastIndex. This tool handles that internally so you always see every match in the input.

Yes. Each match row shows all capture groups as an ordered list. Undefined groups (e.g. from (a)? not matching) show '(undef)'.

The tool caps at 5,000 matches per test to prevent runaway patterns (like zero-width matches) from freezing the page.

Test JavaScript regex with live match highlighting & groups.

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