How do I match special characters like dots, parentheses, or brackets?

Special characters like . ( ) [ ] { } * + ? | \ ^ $ must be escaped with a backslash to match literally. For example, use \. to match a period, \( to match an opening parenthesis, or \ to match a backslash itself.

What's the difference between \d, \w, \s and their negated versions?

\d matches digits (0-9), \w matches word characters (letters, digits, underscore), and \s matches whitespace (spaces, tabs, newlines). Their uppercase versions are negated: \D matches non-digits, \W matches non-word characters, and \S matches non-whitespace.

What's the difference between greedy and lazy quantifiers (* vs *?)?

Greedy quantifiers (* + {n,m}) match as much text as possible, while lazy quantifiers (*? +? {n,m}?) match as little as possible. For example, in the text abc123def, the pattern .* matches the entire string, but .*? matches only until the first opportunity to complete the match.

Why isn't my regex matching across multiple lines?

By default, the dot (.) doesn't match newline characters. Enable the s (dot all) flag to make . match everything including newlines. Also enable m (multiline) if you want ^ and $ to match at line boundaries instead of just the start and end of the entire string.

How do capture groups work?

Capture groups are created with parentheses (). They let you extract specific parts of a match. Reference them in replacements using $1, $2, etc. Named groups use (? ...) syntax and can be referenced as $ in replacements.

How do I test if my regex pattern matches correctly?

Enter your pattern in the regex field and add test text below. Matches will be highlighted in real-time, and the match count will update automatically. Check the Match Details section to see what each capture group matched, and use the Pattern Explanation to understand what your regex does.

What do the different flags mean?

g (global): Find all matches, not just the first. i (ignore case): Case-insensitive matching. m (multiline): ^ and $ match line boundaries. s (dot all): . matches newlines too. u (unicode): Enable full Unicode support.

What are capturing groups and how do I use them in replacements?

Capturing groups use parentheses () to extract parts of a match. In replacements, reference them with $1, $2, etc. For example, the pattern (\w+)@(\w+) matches user@domain and you can swap them with $2@$1 to get domain@user.

What are lookaheads and lookbehinds?

Lookaheads (?=...) and (?!...) check if a pattern follows without including it in the match. Lookbehinds (?<=...) and (?<!...) do the same for patterns before. Example: \d+(?=px) matches numbers followed by px but doesn't include px in the match.

How do I debug regex patterns that aren't working as expected?

Use the Pattern Explanation to understand what each part of your regex does. Check the Match Details to see what's being captured. Start simple and build complexity gradually, testing at each step. Try the Common Patterns library for proven examples similar to what you need.

What regex engine does this tool use?

This tool uses JavaScript's built-in RegExp engine. All patterns and flags available in JavaScript regular expressions are supported, including named capture groups, lookaheads, and lookbehinds (in modern browsers).

Can I share my regex patterns?

Yes! The URL updates automatically with your pattern, flags, and test text. Simply copy the URL from your browser's address bar to share your regex with others. They'll see exactly the same pattern and matches.

Yes! All regex testing happens entirely in your browser using JavaScript. Your patterns and test text never leave your device and are not sent to any server. This tool is safe for testing patterns on sensitive data.

Why is my regex causing catastrophic backtracking?

Catastrophic backtracking occurs when nested quantifiers create exponential combinations, like (a+)+ or (a*)*. Avoid overlapping repetition and use atomic groups or possessive quantifiers when possible. Simplify patterns like (.*).* to just .* to prevent backtracking issues.

Regex Tester & Debugger
Real-Time Pattern Matching

Test, debug, and understand regular expressions with real-time matching, pattern explanation, and common regex library. 100% client-side. Zero data stored.

Regular Expression

/ /

g Global i Ignore case m Multiline s Dot all u Unicode

Test String

Matches 0

Matched text is highlighted in green. The count shows total matches found.

Enter test text above...

Replace

Replace matched text with your string. Use $1, $2 to reference captured groups from your pattern.

Replacement String

Result

Match Details

Shows each match with its position and captured groups. Groups are parts of the pattern in (parentheses).

Enter a pattern to see matches

Pattern Explanation

Enter a pattern to see explanation

Common Patterns

Quick Reference

. Any char \d Digit \w Word char \s Whitespace ^ Start $ End * 0 or more + 1 or more ? Optional {n,m} n to m [abc] Char class () Group | Alternation \b Word boundary

Privacy First

All regex testing happens locally in your browser. Your patterns and test data never leave your device.

Common Use Cases

Email Validation

Test and debug email validation patterns for forms and user input validation across different email formats.

Code Parsing

Extract function names, class definitions, or specific code patterns from source files during development.

Text Processing

Find and replace patterns in logs, documents, or data files with capture groups and complex replacements.

URL Extraction

Extract URLs, domains, or query parameters from web content for scraping and data collection projects.

Log Analysis

Parse server logs, error messages, and debugging output to extract timestamps, error codes, and patterns.

Input Sanitization

Validate and sanitize user input by testing patterns for phone numbers, credit cards, and sensitive data.

Frequently Asked Questions

Why We Built Regex Tester

Regular expressions are incredibly powerful but notoriously difficult to write and debug. A single misplaced character can break your entire pattern, and understanding why it doesn't match can be frustrating. Developers need a fast, reliable way to test regex patterns without context switching between editors and online tools.

Unlike online tools that send your data to external servers, Regex Tester processes everything locally using JavaScript. This means your sensitive data, test strings, and patterns never leave your computer. The tool works completely offline once loaded, making it safe for enterprise environments and air-gapped systems. Test regex on production logs, customer data, or API responses without privacy concerns.

Regex Tester is part of RJL.io's collection of free developer tools - each designed to do one thing exceptionally well, with no accounts, no tracking, and no data collection. Check out our other tools: UUID Generator, QR Code Generator, Base64 Encoder, and more. Every tool is 100% client-side, works offline, and respects your privacy.

Regex Tester & Debugger Real-Time Pattern Matching