What is a regular expression?

A regular expression (regex) is a sequence of characters that defines a search pattern. Regex can match specific strings, validate formats (email, phone), extract data, and perform find-and-replace operations in text.

What regex engine does this tool use?

This tool uses JavaScript's built-in RegExp engine, which follows the ECMAScript regex specification. It's compatible with patterns used in JavaScript, TypeScript, and most modern programming languages.

What are regex flags?

Flags modify how the pattern matches: g (global) — find all matches; i (case insensitive) — match regardless of case; m (multiline) — ^ and $ match line start/end; s (dotAll) — dot matches newlines; u (unicode) — full Unicode support.

How do I match an email address with regex?

A simple email regex: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/. Note that a truly comprehensive email regex is extremely complex — for production, use an email validation library.

What are capture groups?

Parentheses () create capture groups that extract specific parts of a match. (?:...) is a non-capturing group. Named groups (? ...) let you reference captures by name instead of number.

Regex Tester - Test Regular Expressions Online Free

How to Use Regex Tester

1
Enter your regular expression pattern in the "Regex" field.
2
Toggle flags (g, i, m, s, u) using the flag buttons.
3
Enter or paste your test string in the "Test String" area.
4
Matches are highlighted in real time with different colors for each match.
5
See all match details, capture groups, and indices in the results panel.

About Regex Tester

Our free regex tester provides a real-time environment to write, test, and debug regular expressions with instant visual feedback — matches are color-highlighted as you type, with full capture group and flag support.

Key Features

Real-Time Highlighting: Matches highlighted instantly as you type your pattern.
All Regex Flags: Toggle g, i, m, s, u, and y flags with simple buttons.
Capture Groups: See all named and numbered capture group values clearly displayed.
JavaScript Engine: Uses browser RegExp — compatible with JS, TypeScript, and most modern languages.

Who Is This For?

Developers building form validation and data extraction, data engineers parsing log files, QA engineers writing test patterns, and anyone learning or debugging regular expressions.

Privacy & Security

Runs 100% in your browser — no signup, no uploads, no data stored.

Regex Tester