How to test a JavaScript regular expression

1. Enter the pattern

   "Regular expression" takes a JavaScript (ECMAScript) regex source — without the surrounding `/.../`. Up to 2,000 characters. Execution is capped server-side for safety against catastrophic backtracking.

2. Pick flags

   "Flags" accepts any combo of `g i m s u y` (default `g`). `g` = global, `i` = case-insensitive, `m` = multiline anchors, `s` = dotAll, `u` = unicode, `y` = sticky. Up to 12 chars total.

3. Provide test input

   "Test string" is the text to match against. Up to 400,000 characters — plenty for most logs and source files.

4. Press Run

   Result returns the list of matches with capture groups, the full text marked with match positions, and metadata like total match count and time. Catastrophic patterns are aborted with a clear error rather than a server-side hang.