What this does
Test JavaScript regular expressions against a sample string. Matches are highlighted inline, each capture group — numbered or named — is listed separately, and you can switch to replace text with $1-style templates or split the string on the pattern.
Catastrophic backtracking protection
Matching runs in a background worker with a timeout. If a pattern like /^(a+)+$/ takes too long, the worker is stopped automatically and you get a clear error instead of a frozen tab.
Privacy
This runs entirely in your browser. Nothing is uploaded.