JavaScript regex flavour
The tester uses the browser's native RegExp engine — ECMAScript regex. Most features (character classes, quantifiers, anchors, capture groups) match other flavours like PCRE, Python re, and .NET, but a few differ: ECMAScript has named groups via (?<name>...), supports lookbehind, and doesn't have atomic groups.
Watch out for catastrophic backtracking
Patterns like (a+)+b against an input of many as without a terminating b can take exponential time. The tester aborts after 2 seconds to keep your browser responsive — if you hit that, simplify the pattern (avoid nested quantifiers).