Regex tester

No. The conversion runs entirely in your browser. The file never leaves your device – there is no server that could see it. Once the page has loaded, the tool even works offline.

JavaScript's, the same engine your browser uses. That matters: \d and \w behave as in JavaScript, lookbehind is available, but PCRE-only features such as recursion or possessive quantifiers are not. If you are writing the pattern for Python or PHP, test it there too.

No. Some patterns backtrack catastrophically on the wrong input – the classic (a+)+$ against a long line of a's. Matching runs inside a worker with a timeout, so a runaway pattern stops that worker instead of locking up the page. You just get an error and can try again.

At a thousand, to keep the result readable. Patterns that match nearly every character otherwise produce output nobody can scan. If you hit the cap, make the pattern more specific.