Write a pattern (JS flavor, flag chips for g i m s u) and a replacement with $1, $ and $& — only the matched part of each line is replaced, exactly like sed s///, and without g just the first match. Apply per line or to the whole text, interpret \n and \t on demand, and read the live before/after panes with highlighted matches and a changed-line count. Runaway patterns stop after 2 seconds.
Privacy ledger
Data acquired
None.
Libraries used
Lit 3 — bundled into the tool file itself.
Data stored
None.
Data transmitted
None.
Use it on your own page
Download the tool as a single self-contained HTML file: readable source with the Lit library included, zero network requests — it works opened straight from your disk, and you can customize everything.