Regex Tester
Test and validate regular expressions in real time
Match Results
Overview
The Regex Tester is a developer tool that lets you test regular expression pattern matching in real time right in your browser. As you type a regex pattern, matches in the test string are instantly highlighted, and you can view capture groups, match indices, and replacement previews all on one screen. All processing is done entirely via the browser's JavaScript RegExp API, with no data ever sent to a server.
How to Use
Enter a regex pattern
Type your regular expression in the top input field. Toggle flags (g, i, m, s, u, y) as needed.
Enter a test string
Type or paste the text you want to match against. Matches are highlighted in real time.
Review results
Check the match results table for details on each match, including position and capture groups. Enter a replace pattern to preview substitutions.
User Reviews
Tomoya, Frontend Developer
I use this daily for testing form validation regex. The highlighting is intuitive and makes debugging so much easier.
Elena M., Backend Developer
Being able to quickly validate log parsing regex has significantly improved my development workflow.
Misaki, Data Analyst
The preset patterns make it easy even for regex beginners. Love that Japanese phone number patterns are included.
Features
Real-Time Match Highlighting
Matches are highlighted instantly in the test string as you type. Multiple matches are distinguished with alternating colors.
Capture Group Display
Each match's capture groups are displayed with group numbers and named groups, color-coded for clarity.
Replace Preview
Enter a replacement pattern with backreferences like $1 and $2 to preview the substituted result in real time.
Preset Patterns
Quickly load common regex patterns for email, URL, phone number, date, IP address, and more with a single click.
ReDoS Protection
Web Worker and timeout mechanisms prevent browser freezes caused by catastrophic backtracking in regex patterns.
Fully Client-Side
All processing happens in your browser. No input data is ever sent to any server, making it safe for sensitive data.
FAQ
Which regex engine is used?
The tool uses the browser's built-in JavaScript RegExp engine, supporting ECMAScript-compliant regex syntax.
Is my data sent to a server?
No. All processing is done entirely in your browser. Your regex patterns and test strings are never transmitted externally.
What is ReDoS?
ReDoS (Regular Expression Denial of Service) is a vulnerability where certain regex patterns cause exponential backtracking, making processing extremely slow. This tool mitigates this risk with Web Workers and timeouts.
Are named capture groups supported?
Yes. Named capture groups using (?<name>...) syntax are fully supported, and group names and values are displayed in the match results.
What is the maximum input length?
Test strings can be up to 100,000 characters, and regex patterns up to 1,000 characters.