Regex Tester & Debugger Online - Test Regular Expressions Instantly
Test regular expressions online with our free regex tester and debugger. Validate patterns, debug regex, find matches in real-time, and see highlighted results instantly. Perfect for developers working with regex patterns, regex validation, and pattern matching.
Powerful Regex Testing Features
Real-time Testing
See matches appear instantly as you type. No need to click a button—results update automatically as you modify your pattern or test string.
Match Highlighting
All matches are visually highlighted in your test string, making it easy to see exactly what your regex is matching.
Detailed Match Info
View match positions, captured groups, named groups, and full match details for every result found.
All Regex Flags
Toggle Global, Case-insensitive, Multiline, Dot All, Unicode, and Sticky flags to see how they affect matching.
Replace & Transform
Test replacement operations with support for group references ($1, $2) and see results before implementing in code.
Common Patterns
Quick access to pre-built regex patterns for emails, URLs, phone numbers, dates, and more.
How to Use the Regex Tester
Enter Your Pattern
Type or paste your regular expression in the pattern field. Use the Common Patterns menu for quick examples.
Configure Flags
Toggle regex flags (Global, Case-insensitive, etc.) to control matching behavior. Global flag finds all matches, not just the first.
Test Your String
Enter text in the test string field. Matches will highlight automatically, and you'll see detailed match information below.
Use Replace (Optional)
Enter a replacement string to see how your regex would transform the text. Use $1, $2 for captured groups.
Understanding Regular Expressions
What are Regular Expressions?
Regular expressions (regex) are powerful pattern-matching tools used to search, validate, and manipulate text. They're essential for data validation, text parsing, search-and-replace operations, and string processing in programming languages like JavaScript, Python, Java, and more. Learn more about regex fundamentals in the MDN Regex Documentation or explore our other developer tools like the Base64 Decoder and URL Encoder.
Example: Email Validation
Pattern: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
Test: user@example.com
Result: ✅ Match foundCommon Regex Metacharacters
- . - Matches any character except newline
- ^ - Start of string
- $ - End of string
- * - Zero or more of preceding
- + - One or more of preceding
- ? - Zero or one of preceding
- [] - Character class
- () - Capturing group
- | - Alternation (OR)
- \\d - Digit (0-9)
- \\w - Word character
- \\s - Whitespace
Regex Flags Explained
- Global (g): Find all matches, not just the first one
- Case-insensitive (i): Match regardless of letter case
- Multiline (m): ^ and $ match line breaks, not just string start/end
- Dot All (s): . matches newline characters
- Unicode (u): Enable full Unicode support
- Sticky (y): Match only at the position indicated by lastIndex
Best Practices
- Test incrementally: Build complex patterns step by step
- Use anchors: ^ and $ to match entire strings when needed
- Escape special chars: Use \\ to match literal characters like . or *
- Optimize patterns: Avoid catastrophic backtracking with specific quantifiers
- Test edge cases: Try empty strings, special characters, and boundary conditions
- Document patterns: Add comments or explanations for complex regex
For comprehensive regex tutorials and advanced patterns, check out JavaScript.info Regex Guide or Regular-Expressions.info.
Common Regex Use Cases
Form Validation
Validate user input in web forms—email addresses, phone numbers, passwords, credit cards, and other structured data before submission. Use our regex tester to validate patterns before implementing them in your forms. For JSON data validation, try our JSON Formatter tool.
Data Extraction
Extract specific information from logs, documents, or API responses using pattern matching to find dates, IDs, URLs, or other structured data. Combine regex with our CSV to JSON converter for comprehensive data processing workflows.
Text Processing
Clean, transform, or reformat text data by finding and replacing patterns—useful for data migration, content management, and text normalization.
Search & Filter
Implement advanced search functionality that goes beyond simple text matching, allowing users to find content using flexible pattern-based queries.
Regex Tester FAQ
How do I test a regular expression online?
Paste your regex pattern in the pattern field, enter your test string, and the tool will highlight all matches in real-time. Toggle flags to see how they affect matching behavior.
What regex flags are supported?
Our regex tester supports all standard JavaScript regex flags: Global (g), Case-insensitive (i), Multiline (m), Dot All (s), Unicode (u), and Sticky (y). Each flag can be toggled independently.
Can I replace text using regex?
Yes! Enter a replacement string in the "Replace" field and see the result instantly. Use $1, $2, etc. to reference captured groups in your replacement string.
How do I use captured groups?
Wrap parts of your pattern in parentheses to create capturing groups. Access them in replacements using $1 (first group), $2 (second group), etc. Groups are also shown in match details.
What if my regex has an error?
Invalid regex patterns will show an error message explaining the issue. Common problems include unclosed brackets, invalid escape sequences, or malformed quantifiers.
Is my data secure?
Yes! All regex testing happens entirely in your browser. Your patterns and test strings never leave your device, ensuring complete privacy and security.
Can I test regex patterns for different programming languages?
This tool uses JavaScript regex engine, which is similar to regex in many languages. While syntax may vary slightly between languages (JavaScript, Python, Java, etc.), most basic patterns work similarly. Always test in your target language for production code. For JavaScript-specific formatting, try our JavaScript Beautifier.
How do I match special characters in regex?
Escape special characters with a backslash (\\) to match them literally. For example, \\. matches a period, \\* matches an asterisk, and \\+ matches a plus sign. Special characters that need escaping include: . * + ? ^ $ [ ] | ( )
What's the difference between validation and extraction patterns?
Validation patterns use anchors (^ and $) to match entire strings, ensuring the whole input matches the pattern. Extraction patterns omit anchors to find matches anywhere in the text. Use validation for form inputs, and extraction for finding data in larger text blocks.