Regex Tester

What is a Regex Tester?

A regex tester lets you build and debug regular expressions interactively. Type a pattern, paste a sample string, and watch matches highlight in real time. This tool runs entirely in your browser using the standard ECMAScript regex engine (the same one Node.js and every modern browser use), so the matches you see here are exactly what your code will produce.

Features

• Real-time highlighting as you type — no Run button required.
• Flag toggles for g, i, m, s, and u.
• Match details — for each match, see its index, full text, and every capture group (positional and named).
• Common patterns — one-click presets for email, URL, ISO date, IPv4, UUID.
• Cheat sheet sidebar for quick reference.
• Web Worker execution for complex patterns to keep the UI responsive.

Performance tips

Catastrophic backtracking can hang any regex engine. If a pattern with nested quantifiers (like (a+)+) seems to lock up the page, the worker will cancel it after a timeout and report the issue. Prefer atomic groups, possessive quantifiers, or simpler structures.