Regex Tester FAQ

Q: Which Regex engine does this use? PCRE?

A: This tool runs directly in your browser using the built-in JavaScript (ECMAScript) regex engine. It does not support some PCRE-specific features (like recursion), but it is fully compatible with JS/TS development environments. Patterns tested here are safe to use in your web projects.

Q: Is Lookbehind supported?

A: Yes. Modern browsers (Chrome 62+, Firefox 78+, Safari 16.4+) support JavaScript Lookbehind syntax (?<=...) and (?<!...). If you are using an older browser, you might see an invalid group error.

Q: Why doesn't . (dot) match newlines?

A: By default, the dot matches any character except line terminators. To match newlines as well, you can:

1. Use [\s\S] instead of ..
2. Enable the Single Line (s) / DotAll flag in the options panel (if supported).

Q: How large text can it handle?

A: Performance depends on your CPU as matching runs locally. It can handle tens of thousands of lines in milliseconds for simple patterns. However, generally avoid "Catastrophic Backtracking" patterns as they can freeze your browser tab.