Which regex flavor does this use?

Standard JavaScript regex (ECMAScript). Most Python, Java, and PCRE patterns work, but some lookaround and Unicode features differ.

How do I avoid catastrophic backtracking?

Avoid nested quantifiers like (a+)+ and use atomic groups or possessive quantifiers where supported. Test on pathological inputs.

Yes. Use \1 or \k in the pattern. The tool shows the matched group values.

Testers

Test regular expressions with live match highlighting, capture group inspection, and a handy cheatsheet. Everything runs in your browser.

Pattern

//g

Flags

Test String

Match Highlighting

Matches appear here...