Is this regex tester free to use?

Yes. Testing and debugging regular expressions is completely free, with no limits or sign-up required.

Which regex flavor does this tool use?

This tool runs your pattern through the browser's native JavaScript regex engine, so results match exactly what you'll get in Node.js or any browser console.

Is my test data sent anywhere?

No. Your pattern and test string are matched entirely in your browser — nothing is uploaded or stored.

Why does my regex work here but not in Python or Go?

Every language's regex engine has its own quirks. This tool uses JavaScript's flavor, so features like lookbehind support, named groups, and Unicode property escapes can behave differently in Python's `re`, Go's `regexp`, or PCRE. Always test against the engine your code actually runs on.

What do the g, i, m, s, and u flags mean?

g finds all matches instead of stopping at the first, i ignores case, m makes ^ and $ match line starts and ends, s lets . match newlines, and u enables full Unicode mode.

Why does my pattern with * or + freeze other testers?

Patterns that can match an empty string (like /a*/g) or that backtrack exponentially on certain input can hang a naive matcher. This tool caps matching at 100,000 results and always advances past zero-length matches, so it stays responsive even on tricky patterns.