RegEx Tester & Debugger

Test regular expressions in real-time with syntax match highlighting and capture group extraction.

Common Presets:

Regular Expression (RegEx)

/ /

g (global) i (ignore case) m (multiline) s (dotall)

Test String 0 matches

Highlighted Match Preview

How to Use this Tool

1. Enter your regular expression pattern and select regex flags (g, i, m). 2. Enter your test string or sample text. 3. View highlighted matches, capture groups, and match counts in real-time.

Mathematical Formula & Explanation

RegExp Matching & Group Extraction: 1. Pattern Compilation: Compile pattern and active flags: new RegExp(pattern, flags). 2. Global Iteration: While regex.exec(text) !== null, capture: - Match String: result[0] - Start & End Indices: result.index to (result.index + result[0].length) - Capture Groups: result.slice(1) 3. Visual Overlay: Splice match intervals into highlighted HTML spans while escaping HTML entities.

About RegEx Tester & Debugger

The Free Online RegEx Tester & Debugger is an interactive development tool that lets you build, test, and debug regular expressions in real-time with instant match highlighting and capture group extraction.

Writing regular expressions is notoriously tricky. A misplaced bracket or greedy quantifier can result in catastrophic backtracking or missed matches. This tester provides immediate visual feedback to verify your regex patterns against real sample data before putting them in production.

Key Features & Capabilities:
• Real-Time Match Highlighting: Instantly visualizes matching substrings as you type your pattern or sample text.
• Complete Flag Support: Toggle standard regex flags including Global (`g`), Case-Insensitive (`i`), and Multiline (`m`).
• Capture Group Inspection: Displays detailed breakdowns of parenthesized capture groups ($1, $2) and their match indices.
• Match Counter: Provides exact counts of total occurrences found throughout the target text.
• High-Performance Engine: Built on modern JavaScript RegExp runtime for zero-delay execution and instant feedback.

Common Applications:
1. Form Validation: Test regex patterns for email validation, phone numbers, strong passwords, and credit card numbers.
2. Data Extraction & Web Scraping: Verify capture groups for extracting URLs, dates, prices, or product codes from messy text.
3. Log Parsing & Search-and-Replace: Develop search patterns for Apache/Nginx access logs, server metrics, and CSV cleaning.

Frequently Asked Questions (FAQs)

4 Questions

Everything you need to know about RegEx Tester & Debugger and how it works.

Is RegEx Tester & Debugger completely free to use?

Yes, RegEx Tester & Debugger is 100% free with unlimited calculations. There are no registration requirements, no paywalls, and no hidden subscriptions.

Is my data private and secure while using RegEx Tester & Debugger?

Yes, your privacy is fully protected. All calculations run strictly client-side within your web browser. None of your input data is stored or logged on our web servers.

Can I use RegEx Tester & Debugger on my mobile smartphone or tablet?

Yes, this tool is fully responsive and optimized for mobile devices, tablets, and desktop computers. You can also install it as a Progressive Web App (PWA) directly to your home screen.

How accurate are the results provided by RegEx Tester & Debugger?

All calculations and transformations follow standard mathematical algorithms and authoritative standards to provide real-time, high-precision results.