Regex Testing Lab
g - Global
Find all matches
i - Ignore Case
Case-insensitive matching
m - Multiline
^ and $ match line boundaries
s - Dotall
. matches newlines
u - Unicode
Unicode support
y - Sticky
Match from lastIndex
Quick Reference Cheatsheet
Practical Examples
Complete User Guide
How to Use This Tool
- Step 1: Enter your regular expression pattern in the "Regular Expression Pattern" field
- Step 2: Select appropriate flags by clicking on them (g, i, m, s, u, y)
- Step 3: Enter or paste your test string in the "Test String" area
- Step 4: Click "Test Regex" to see matches and detailed results
- Step 5: View highlighted matches, match count, and position information
- Step 6: Use the Quick Reference Cheatsheet to learn regex syntax
- Step 7: Try practical examples by clicking on them to auto-fill the form
Understanding Your Results
When you test a regex pattern, the tool provides:
- Highlighted Matches: All matched text is highlighted in black with white text
- Match Count: Total number of matches found in your test string
- Match Details: Each match shows the matched text, its position (index), and length
- Statistics: Visual cards displaying total matches, pattern length, and test string length
How Results Are Calculated
The tool uses JavaScript's native RegExp engine:
- Pattern Compilation: Your pattern is compiled with selected flags (e.g., new RegExp(pattern, "gi"))
- Match Execution: The exec() or match() method finds all occurrences
- Position Tracking: Each match index is recorded for precise location
- Length Calculation: Match length is measured in characters
- Highlighting: Original text is split and matches are wrapped in span elements
Interesting Facts About Regex
- Origin: Regular expressions were invented by mathematician Stephen Cole Kleene in 1951
- Universal: Regex is supported in virtually every modern programming language
- Powerful: A single regex pattern can replace hundreds of lines of string manipulation code
- Performance: Well-written regex is highly optimized and extremely fast
- Validation: Over 80% of form validation on the web uses regular expressions
- Search Engines: Google and other search engines use regex for advanced search features
- Security: Regex is crucial for input validation and preventing injection attacks
- Text Editors: All major text editors and IDEs use regex for find-and-replace operations
Additional Tips & Best Practices
Performance Tips
- Use specific patterns instead of greedy quantifiers (.*) when possible
- Avoid excessive backtracking by using atomic groups and possessive quantifiers
- Place frequently matching alternatives first in alternation (|)
- Use anchors (^ and $) to limit search scope when appropriate
Security Tips
- Always validate user input with regex before processing
- Be careful with ReDoS (Regular Expression Denial of Service) attacks
- Use non-capturing groups (?:) when you don't need to capture text
- Test regex patterns with edge cases and malicious input
Writing Better Patterns
- Use character classes [a-z] instead of multiple alternations (a|b|c|...)
- Comment complex regex using the x flag (in languages that support it)
- Break complex patterns into smaller, testable pieces
- Use online regex testers during development
- Document your regex patterns with comments in your code
Real-World Use Cases
Form Validation
- Email address validation for contact forms
- Phone number format verification
- Password strength checking (uppercase, lowercase, numbers, symbols)
- Credit card number validation
- ZIP code and postal code validation
Data Extraction
- Extracting URLs from HTML or plain text
- Parsing log files for specific patterns
- Extracting phone numbers from documents
- Scraping email addresses from web pages
- Finding dates in various formats
Text Processing
- Search and replace operations in code editors
- Data cleaning and normalization
- Removing unwanted characters or whitespace
- Converting text formats (CSV to JSON, etc.)
- Masking sensitive information (SSN, credit cards)
Development Tools
- Syntax highlighting in code editors
- URL routing in web frameworks
- Command-line argument parsing
- Configuration file validation
- API parameter validation
About This Tool
JavaScript Regex Cheatsheet Tool is a comprehensive, production-ready web application designed to help developers, students, and professionals master regular expressions through interactive learning and testing.
Features
- Real-time Testing: Test regex patterns instantly with live feedback
- Visual Highlighting: See matches highlighted directly in your test text
- Comprehensive Cheatsheet: Quick reference for all regex syntax and patterns
- Practical Examples: Ready-to-use patterns for common tasks
- Flag Support: Full support for all JavaScript regex flags (g, i, m, s, u, y)
- Detailed Results: Match count, positions, and lengths clearly displayed
- Responsive Design: Works perfectly on desktop, tablet, and mobile devices
- SEO Optimized: Comprehensive meta tags and semantic HTML structure
Who Is This For?
- Web Developers: Testing and validating form inputs, URLs, and data
- Backend Developers: Processing logs, parsing data, and validating API inputs
- Data Scientists: Cleaning and extracting information from text data
- Students: Learning regular expressions through interactive examples
- DevOps Engineers: Parsing configuration files and log analysis
- Content Managers: Bulk text processing and content validation
Technology Stack
- HTML5: Semantic markup for accessibility and SEO
- CSS3: Modern styling with responsive design and animations
- Vanilla JavaScript: No dependencies, fast and lightweight
- Font Awesome: Professional icon library
- Native RegExp API: Leverages JavaScript's built-in regex engine
| Feature | Details |
|---|---|
| Price | Free |
| Rendering | Client-Side Rendering |
| Language | JavaScript |
| Paywall | No |
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