Regex Tester

What Is a Regex Tester?

A regex tester is a developer tool for building, testing, and debugging regular expressions — powerful pattern-matching strings used in programming, text processing, data validation, and search operations. Regular expressions (regex) can match complex text patterns like email addresses, phone numbers, URLs, dates, and custom formats. A tester provides instant visual feedback, highlighting matches in your test string as you build your pattern.

Why Use a Regex Tester?

Writing regex without a tester is like writing code without a debugger — possible but painful. Regex syntax is dense and a single character can completely change the meaning of a pattern. A tester shows you immediately what your pattern matches (and what it doesn't), displays capture groups, and helps you understand complex patterns through real-time highlighting. It's an essential tool for developers working with form validation, log parsing, data extraction, or text transformation.

How Does This Tester Work?

Enter a JavaScript regular expression and a test string. The tool highlights all matches in real-time as you type, displays capture groups and their values, and shows match positions. Toggle regex flags (global, case-insensitive, multiline, dotAll, unicode, sticky) with one click. Use substitution mode to test find-and-replace operations. A built-in cheatsheet provides quick reference for regex syntax without leaving the page.

Is It Safe?

Yes. All regex testing happens locally in your browser using JavaScript's native RegExp engine. Your test strings and patterns are never sent to any server. The tool also includes a 2-second timeout to protect against catastrophic backtracking — a common regex pitfall that can freeze other tools.

What regex flavor does this support?

JavaScript (ECMAScript) regular expressions. This includes all modern features like named groups, lookbehind, unicode properties, and the dotAll flag.

Does it support find and replace?

Yes. Toggle substitution mode to enter a replacement pattern and see the result. Supports backreferences ($1, $2) and named backreferences ($<name>).

What if my regex is too slow?

The tool has a 2-second timeout to protect against catastrophic backtracking. If your pattern takes too long, you'll see a warning instead of freezing the browser.

Is my data safe?

Yes. All regex testing happens locally in your browser. Your test strings and patterns are never sent to any server.