Regex TesterTest regular expressions with live matching, highlighting, and group extraction.

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Regex Tester

Test regular expressions with live matching, highlighting, and group extraction.

How to Use

Enter Pattern

Type your regular expression pattern.

Set Flags

Toggle flags: global (g), case-insensitive (i), multiline (m), dotall (s).

Test String

Enter or paste the test string to match against.

What Is Regex Tester?

The Regex Tester is an interactive tool for testing and debugging regular expressions. Enter a regex pattern and a test string to see matches highlighted in real time. It displays match details including full match text, index position, numbered capture groups, and named capture groups. Supports JavaScript regex syntax with global (g), case-insensitive (i), multiline (m), and dotall (s) flags. The tool provides instant visual feedback with highlighted matches in the test string.

Why Use Our Regex Tester?

Test regex patterns with instant visual feedback
See capture groups and named groups for each match
Debug complex regular expressions interactively
Copy the complete regex pattern with flags

Common Use Cases

Development

Test and debug regex patterns before using them in code.

Data Extraction

Verify that patterns correctly match and capture the desired data.

Validation

Test input validation patterns against sample data.

Learning

Learn regex by experimenting with patterns and seeing results in real time.

Technical Guide

The tester creates a new RegExp object from the pattern and flags on each input change. With the global flag, it uses a while loop with regex.exec() to find all matches, advancing lastIndex after zero-length matches to prevent infinite loops. Each match result includes the full match (match[0]), positional index, numbered groups (match.slice(1)), and named groups (match.groups). For highlighting, a separate pass through the string wraps matched portions in <mark> elements with Tailwind styling. The highlighted output uses dangerouslySetInnerHTML with HTML entity escaping for safety. Invalid regex patterns are caught and displayed as error messages instead of crashing the component.

Tips & Best Practices

1
Toggle the global (g) flag to find all matches vs. just the first
2
Named groups use (?<name>pattern) syntax
3
The dotall (s) flag makes . match newline characters
4
Invalid regex patterns show clear error messages

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Frequently Asked Questions

QWhich regex flavor is used?

JavaScript (ECMAScript) regular expressions. Features like lookbehind (available in modern browsers) are supported.

QWhat are capture groups?

Groups defined with parentheses () capture matched text. Named groups use (?<name>pattern) syntax.

QWhat does the global (g) flag do?

Without g, only the first match is found. With g, all matches in the string are found.

QWhy does my pattern cause an error?

Common causes: unescaped special characters, mismatched brackets, or unsupported syntax.

QCan I test multiline patterns?

Yes, enable the multiline (m) flag so ^ and $ match line boundaries, and dotall (s) so . matches newlines.

About Regex Tester

Regex Tester is a free online tool from FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration required. No ads. Just fast, reliable tools.