What regex flavor does this tester use?

The tester uses JavaScript's built-in RegExp engine, which follows the ECMAScript specification. This means the behavior you see matches exactly what runs in Node.js and browsers. Some advanced features like lookbehind assertions are supported in modern engines.

What do the g, i, and m flags do?

The g (global) flag finds all matches in the string instead of stopping at the first. The i (case-insensitive) flag makes the pattern match both uppercase and lowercase letters. The m (multiline) flag makes ^ and $ match the start and end of each line instead of the entire string.

Why is my regex not matching?

Common causes: missing escape characters for special chars like . * + ? [ ] ( ) { } | ^ $, forgetting the global flag when you expect multiple matches, or character class issues. The tester highlights matches in real time, so try simplifying your pattern and adding complexity gradually to isolate the issue.

Can I test regex patterns for other languages like Python or PHP?

The tester uses JavaScript's regex engine, which shares most syntax with PCRE (Python, PHP, Ruby) but has some differences. Named groups use (? ...) syntax in modern JavaScript. Backreferences and lookaheads work, but some PCRE features like conditional patterns are not supported.

What are capture groups and how do I use them?

Capture groups are parts of your regex enclosed in parentheses. They let you extract specific portions of the matched text. For example, (\d{4})-(\d{2})-(\d{2}) captures the year, month, and day separately. The tester highlights each group in a different color and lists them below the match.

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Regex Tester

A regex tester lets you test regular expressions against a string with real-time match highlighting. It supports capture groups, and flags like global (g), case-insensitive (i), and multiline (m). Matches update instantly as you type.

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What is Regex Tester?

A regex tester is a tool that evaluates regular expressions against test strings in real time, highlighting matches and capture groups visually, and supporting standard flags like global, case-insensitive, and multiline.

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These tools work great alongside Regex Tester:

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Start Using Regex Tester Today

Regular expressions are powerful but notoriously hard to get right on the first try. This regex tester closes the feedback loop by showing you exactly what your pattern matches — and what it doesn't — the instant you type. With color-coded capture groups, real-time flag toggling, and a JavaScript-native engine, you can debug patterns in seconds instead of deploying broken regex to production and finding out the hard way.

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