Regex Tester & Debugger

A faster loop for writing regular expressions

Nobody writes a non-trivial regex correctly on the first try. The productive workflow is iterative: paste representative text, write the smallest pattern that matches something, then tighten it while watching the highlights — every false positive and missed match is visible immediately instead of surfacing in production logs.

Capture groups deserve the same scrutiny as the overall match. The match table shows numbered groups and named groups per match, so extraction patterns — pulling the domain from emails, the timestamp from log lines, the version from user agents — can be validated field by field before they ship.

Flags, briefly

g finds all matches instead of the first; i ignores case; m makes ^ and $ match per line; s lets . cross newlines; u enables full Unicode handling (and stricter syntax); y anchors each match at the exact position the last one ended. The tester forces a global scan for listing and highlighting — your saved flag combination is what the Replace tab and your production code will use, drawing on the same RegExp engine your JavaScript runs on.

Worked example

Five patterns worth keeping within reach, each written exactly as you would type it into the pattern field — no surrounding slashes — and run with the g flag against the test string beside it. The Matches column lists the literal text each pattern found, so you can paste any row into the tester above and reproduce it keystroke for keystroke.