Which regex engine does this use?

This tool uses JavaScript's built-in RegExp engine (as implemented in your browser). It supports most standard regex features including lookaheads, named groups, and Unicode escapes, but does not support PCRE-specific features like lookbehinds in older browsers.

Why do I see no matches?

Check that the g flag is enabled if you expect multiple matches. Also verify the case - the i flag disables case-sensitivity. Use one of the built-in examples to verify the tool is working, then adapt your pattern.

What are capturing groups?

Capturing groups are parts of a pattern surrounded by parentheses, e.g. (\w+). The text matched by each group is captured separately and shown in the match details panel below the preview.

My regex makes the browser slow - what should I do?

This tool stops execution after 500ms to protect against catastrophic backtracking. If your pattern times out, try making it more specific. Common causes include patterns like (a+)+ or (.*)*, which can cause exponential backtracking.

Can I use this for Python or PCRE regex?

This tester uses JavaScript regex syntax. Most patterns are compatible, but some PCRE features like (?P ...) named groups, conditional patterns, or possessive quantifiers are not supported in JavaScript.

Regex Tester

Test JavaScript regular expressions with real-time match highlighting, capturing group display, and flag toggles. Everything runs in your browser.

Client-Side Privacy Free Forever Mobile & PC

Quick examples:

Regular expression

Global - find all matches

Test string

What is a regex tester?

A regex tester (regular expression tester) lets you write a pattern and test it against a sample string to see which parts match. This tool uses JavaScript's built-in RegExp engine. Matches are highlighted in the preview, and capturing group values are displayed below.

Supported flags

g - Global: find all matches, not just the first one.
i - Case-insensitive: A matches both A and a.
m - Multiline: ^ and $ match the start and end of each line.
s - Dotall: . matches newline characters too.
u - Unicode: treats the pattern and string as Unicode code points.

Common regex patterns

\d+ - one or more digits
\w+ - one or more word characters (letters, digits, underscore)
\s+ - one or more whitespace characters
^.+$ - any non-empty line (with the m flag)
(?:...) - non-capturing group
(?<name>...) - named capturing group

Catastrophic backtracking protection

Some regular expressions - particularly those with nested quantifiers - can cause exponential backtracking that freezes the browser. This tool stops execution after 500 milliseconds and reports a timeout, protecting your browser. If this happens, simplify your pattern, use possessive quantifiers (not available in JavaScript), or restructure the regex to avoid ambiguous paths.

Privacy & Security

All matching runs entirely client-side in your browser. Your pattern and test text are never sent to any server.

Our Tool Features

Why developers and engineers prefer this tool for everyday development workflows.

Cross-Device Compatibility

Works seamlessly on all modern desktop, tablet, and mobile browsers including Chrome, Firefox, Safari, Edge on Windows, macOS, Linux, Android, and iOS.

Fast In-Browser Execution

Processes your data in real-time directly inside your browser. No queue wait times, no network round-trips, and zero latency.

100% Client-Side Privacy

Your code, tokens, passwords, and files are never transmitted to any external server or saved in logs. Everything runs locally in memory.

Instant File & Clipboard Input

Supports one-click sample loading, file drag-and-drop, and pasting directly from your clipboard (Ctrl + V) for effortless workflows.

Strict Standards Compliant

Built strictly according to official specifications (RFCs, W3C, ISO, and standard cryptographic algorithms) ensuring accurate output every time.

Unlimited Free Usage

No registration, no accounts, no subscriptions, and no hidden daily quotas. Use this tool as many times as you need without restrictions.

Recent Updates & Improvements

Continuously maintained and improved based on developer feedback.

Client-Side Speed & Performance UpgradeRecent Update

Optimized processing algorithms with Web Workers and native browser APIs (SubtleCrypto, Canvas 2D) for instantaneous zero-latency execution.

Clipboard Paste & Drag-and-Drop AddedFeature Added

Added instant clipboard pasting (Ctrl + V) and drag-and-drop file support to accelerate developer productivity without manual downloads.

Enhanced Mobile & Touch Device ExperienceUI/UX Update

Redesigned responsive touch controls, flexible copy buttons, and camera preview framing for mobile and tablet browsers.

Frequently Asked Questions

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