Regex Tester - Free Online JavaScript Regular Expressions Tool

Using the Online Regex Debugger

Type your regex pattern between the slashes.
Select your flags (g, i, m, s) to control matching behavior.
Input your test string to see live regex highlighting.
Check the capture groups section for detailed results.

Regex Tester: debug and refine your JavaScript regular expressions instantly with our high-performance regex tool. Built on the browser's native engine, this utility provides real-time feedback and live regex highlighting, ensuring that your patterns behave exactly as they will in your production code. Whether you're a seasoned developer or just learning pattern matching, our tool offers a clean, terminal-style interface for rapid prototyping and debugging.

Advanced Regex Highlighter and Matcher

Our generator excels at breaking down complex patterns, offering a clear view of both numbered and named capture groups for every match. This online regex debugger surfaces engine-level errors immediately, helping you troubleshoot syntax issues without guess work. By processing all data locally, we guarantee that your sensitive test strings remain private while delivering the speed of a native application. Use it to perfect your validation logic, data extraction, or text transformation patterns with ease.

Live regex highlighting with support for all standard JavaScript flags
Detailed capture group breakdown including modern named group syntax
Native RegExp engine behavior for 1:1 parity with your software
Local-first design for immediate performance and data security

Frequently Asked Questions

Which regex flavour does this tester use?

It uses the browser's native JavaScript RegExp engine (ECMA-262). Patterns and flag behaviour will match exactly what runs in Node.js, Chrome, Firefox and Safari - but not PCRE, Python, or .NET regex.

Are lookbehinds supported?

Yes. Modern browsers (Chrome 62+, Firefox 78+, Safari 16.4+) support both positive (?<=…) and negative (?

Why are my capture groups showing as undefined?

A group can be undefined when it sits inside an alternation that didn't match, or after a quantifier that matched zero times. The tool shows the literal string <undefined> in those slots so you can spot the difference from an empty string.

How do I match across multiple lines?

Use the m flag so ^ and $ match at every line break, or the s (dot-all) flag so . matches newlines. The two are independent - pick the one that fits the pattern you're writing.

Perfect your pattern matching today with this professional-grade utility. Explore more Code tools at Chunky Munster and make this Regex Tester your go-to tool for all regular expression development.

Regex Tester online

Test regular expressions with live match highlighting and group capture