Online Regex Tester & Debugger
Test your regular expressions against sample text. See matches highlighted in real-time and explore match details.
Match Information
Matches will be listed here.
About Regular Expressions (Regex)
A regular expression is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation. Regular expressions are a powerful tool for text processing and manipulation.
How this Tool Works:
- Enter Pattern & Flags: Type your regex pattern and any desired flags (like 'g' for global, 'i' for case-insensitive).
- Provide Test String: Paste the text you want to test your regex against.
- Real-time Results: As you type, the tool attempts to execute the regex using JavaScript's built-in regex engine.
- Highlighted Matches: The test string area will show highlighted matches if found.
- Match Information: A table below will list all found matches, including their index, the matched text, and any capturing groups.
- Error Handling: If your regex pattern is invalid, an error message will be displayed.
- Cheat Sheet: A handy regex cheat sheet can be toggled for quick reference to common syntax.
Flavor Note: This tool primarily uses JavaScript's regular expression engine. While many patterns are compatible across different programming languages (like PHP/PCRE, Python), there can be subtle differences in syntax or feature support. Always test your regex in the specific environment where it will be used.
About Regex Tester
What does this tool do?
Our Online Regex Tester provides real-time regular expression testing with JavaScript engine support, live match highlighting, detailed capture group analysis, and an integrated cheat sheet. It validates patterns instantly and shows comprehensive match information for debugging and development workflows.
Why is this useful?
Regular expressions are powerful but complex. Testing patterns before implementation prevents bugs, saves development time, and ensures accuracy. Our tool provides immediate feedback, making regex development faster and more reliable than trial-and-error coding.
Who uses this tool?
- Software developers - Test patterns for form validation, data parsing, and text processing
- Data analysts - Extract and clean data from text files and logs
- System administrators - Parse log files and configuration data
- QA engineers - Validate input patterns and test edge cases
- Students and educators - Learn and teach regular expression concepts
How to test regular expressions
- Enter your regex pattern in the pattern field (without surrounding slashes)
- Add flags like 'g' for global matching or 'i' for case-insensitive
- Paste your test text in the test string area
- View real-time highlights showing matches in your text
- Examine match details including position and capture groups
- Toggle the cheat sheet for quick syntax reference
Example
Testing email pattern `/[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}/g` against "Contact: john@example.com or jane@test.org" highlights both email addresses and shows their positions and full matches.
Understanding regex results
- Match Highlighting
- Yellow highlights in the test text show exactly what your pattern matched. Essential for visual verification of pattern accuracy.
- Match Index
- Starting character position of each match in the test string. Useful for programmatic text replacement and extraction.
- Capture Groups
- Parentheses in patterns create groups that extract specific parts of matches. Numbered from 1, they appear as separate columns.
- Global Flag (g)
- Finds all matches instead of stopping at the first. Essential for processing multiple occurrences in text.
- Error Messages
- Red alerts show syntax errors in your pattern. Common issues include unescaped special characters and unclosed groups.
Effective regex patterns
- Specific patterns: Match exactly what you need, avoid over-broad matches
- Anchored patterns: Use ^ and $ to match from start/end when needed
- Escaped special chars: Properly escape . * + ? ^ $ { } ( ) | [ ] \
- Non-greedy quantifiers: Use *? and +? to match minimally when appropriate
Common regex pitfalls
- Catastrophic backtracking: Complex patterns can cause infinite loops
- Greedy matching: Default quantifiers match as much as possible
- Case sensitivity: Patterns are case-sensitive unless 'i' flag is used
- Language differences: JavaScript regex differs from PHP, Python, etc.
Best practices for regex development
- Start simple and build complexity gradually
- Test with edge cases including empty strings and special characters
- Use non-capturing groups (?:) when you don't need the capture
- Comment complex patterns for future maintenance
- Consider readability - sometimes multiple simple patterns beat one complex one
Common use cases
Form Validation
Test patterns for email addresses, phone numbers, passwords, and postal codes before implementing in web forms to ensure proper user input validation.
Data Extraction
Extract structured data from logs, CSV files, or unstructured text by testing patterns against sample data before running on large datasets.
Text Processing
Develop patterns for find-and-replace operations, text cleanup, and format standardization with immediate visual feedback.
Learning and Debugging
Understand regex behavior, debug complex patterns, and explore advanced features like lookaheads and capture groups with real-time feedback.
Regex engine compatibility
This tool uses JavaScript's regex engine, which supports most common features but may differ from PCRE (PHP), Python, or .NET engines in advanced features like lookbehinds and atomic groups.
Performance considerations
Complex patterns with nested quantifiers can cause performance issues. Test with realistic data sizes and consider alternative approaches for very large text processing tasks.