RegexLab

Regex pattern tester, explainer, and debugger. Three modes: test (match pattern against text or file with results), explain (plain-English breakdown of any regex pattern), and library (15 built-in patterns for email, URL, IPv4, phone, date, UUID, JWT, semver, and more). Detects common regex mistakes like greedy .* without lazy modifier. Supports ignore-case and multiline flags. Match results show positions and named groups. JSON and text output. Zero dependencies — single Python file.