Which regex flavor does this tester use?
JavaScript's native RegExp engine — the same one used by Node.js and every modern browser.
Is my test string or pattern uploaded anywhere?
No, matching and highlighting run entirely in your browser using JavaScript's built-in regex engine.
Can I preview a find-and-replace before applying it?
Yes, type a replacement pattern (including $1, $2 group references) and the result updates live below.