Test a regular expression against your own text and see every match with the groups inside it, colour-coded by the part of the pattern that took them. Rewrite the matches with JavaScript’s own replacement syntax, read the pattern back as a railroad diagram or in plain English, and measure how long it takes as the text gets longer, to catch catastrophic backtracking (ReDoS) before it reaches production. Supports the g, i, m, s and u flags, including \p{…} property escapes. Free, sandboxed, and running entirely in your browser.