Regex Tester
Free online Regex Tester. Simple, fast, and secure tool running in your browser.
What is the Regex Tester?
The Regex Tester is a browser-based regular expression testing tool that lets developers write, test, and debug regex patterns in real time. Regular expressions (regex) are powerful pattern-matching sequences used across nearly every programming language for text validation, data extraction, string manipulation, and search operations.
This tool provides instant visual feedback as you type — highlighting matches, displaying capture groups, showing match indexes, and even supporting find-and-replace operations. All processing happens entirely in your browser using JavaScript's native RegExp engine, so your test data never leaves your device.
How to Use the Regex Tester
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1
Enter Your Regex Pattern
Type your regular expression between the forward slashes. Add flags like
g(global),i(case-insensitive), orm(multiline) in the flags field. -
2
Paste Your Test String
Enter the text you want to test against. Matches are highlighted in real time as you type.
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3
Review Matches and Capture Groups
Each match is shown with its index position, full match value, and any capture groups. Copy individual matches or all matches at once.
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4
Use Replace Mode (Optional)
Switch to Replace mode to test find-and-replace operations. Use
$1,$2, etc. to reference capture groups in your replacement string.
Real-World Use Cases
Email Validation
Test patterns like [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} to validate email addresses in form inputs before production deployment.
Log File Parsing
Extract timestamps, error codes, or IP addresses from server logs using capture groups. Test your patterns here before coding them into your pipeline.
Data Cleaning
Use Replace mode to strip HTML tags, normalize whitespace, or reformat phone numbers and dates in bulk text data before import.
Frequently Asked Questions
What regex engine does this tool use?
This tool uses JavaScript's built-in RegExp engine, which supports ECMAScript regular expressions. This is the same engine used in Node.js, Chrome, Firefox, and all modern browsers.
What are capture groups?
Capture groups are parts of a regex enclosed in parentheses. They let you extract specific portions of a match. For example, in the pattern (\d{4})-(\d{2})-(\d{2}), Group 1 captures the year, Group 2 the month, and Group 3 the day.
What flags are supported?
Common flags include: g (global — find all matches), i (case-insensitive), m (multiline), s (dotAll — dot matches newlines), and u (Unicode).
Is my test data sent to any server?
No. All regex processing happens entirely in your browser using client-side JavaScript. Your test strings and patterns never leave your device.
Can I use this to test regex for Python or PHP?
Most basic regex patterns work across languages. However, some advanced features like lookbehind assertions or named groups may differ between engines. This tool tests against the JavaScript regex engine specifically.