Regex Tester

About Regular Expression Testing

Regular expression testing is essential for developing, debugging, and validating regex patterns. Our regex tester provides real-time matching, detailed explanations, and comprehensive testing capabilities to ensure your patterns work correctly.

Real-time pattern matching and validation
Detailed match results with capture groups
Pattern explanation and breakdown
Multiple test strings and batch testing
Support for different regex flavors and flags

Testing Features

Match Analysis

Live match highlighting
Capture group extraction
Match count and positions
Non-matching text identification
Performance timing information

Pattern Validation

Syntax error detection
Pattern explanation
Optimization suggestions
Common pitfall warnings
Cross-platform compatibility

Frequently Asked Questions

Why isn't my regex pattern matching?

Common issues include incorrect escaping, missing anchors, case sensitivity, or greedy vs lazy quantifiers. Use the tester to see exactly where matches fail and adjust your pattern accordingly.

How do I test multiline patterns?

Use the multiline flag (m) to make ^ and $ match line boundaries instead of string boundaries. The global flag (g) will find all matches, not just the first one.

What are the different regex flavors?

Different languages have slight variations: JavaScript (ECMAScript), Python (PCRE), Java, .NET, etc. Most common patterns work across flavors, but advanced features may differ.

How can I optimize slow regex patterns?

Avoid excessive backtracking by using atomic groups, possessive quantifiers, and being specific with character classes. Test with large inputs to identify performance issues.

Testing Examples

Email Validation Test:

Pattern: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/

Test Strings:

john@example.com ✓ Match

user.name+tag@domain.co.uk ✓ Match

invalid.email ✗ No match

@domain.com ✗ No match

Phone Number Extraction:

Pattern: /$?([0-9]{3})$?[-. ]?([0-9]{3})[-. ]?([0-9]{4})/g

Test String:

Call us at (555) 123-4567 or 555.987.6543 for support.

Matches found:

1. (555) 123-4567 - Groups: 555, 123, 4567

2. 555.987.6543 - Groups: 555, 987, 6543

Regex Flags and Modifiers

Common Flags

g - Global

Find all matches, not just the first one

/pattern/g

i - Ignore Case

Case-insensitive matching

/pattern/i

m - Multiline

^ and $ match line boundaries

/pattern/m

Advanced Flags

s - Dot All

. matches newline characters

/pattern/s

u - Unicode

Enable Unicode features

/pattern/u

y - Sticky

Match only at lastIndex position

/pattern/y

Testing Best Practices

Test Edge Cases: Empty strings, very long strings, special characters
Use Multiple Examples: Test both matching and non-matching cases
Check Performance: Test with large inputs to identify bottlenecks
Validate Capture Groups: Ensure groups capture the expected content
Cross-Platform Testing: Test patterns in target environments
Document Test Cases: Keep examples for future reference
Gradual Building: Start simple and add complexity incrementally

Testing in Code

JavaScript Testing:

// Test regex pattern
function testRegex(pattern, testStrings, flags = '') {
  const regex = new RegExp(pattern, flags);
  
  testStrings.forEach(str => {
    const matches = str.match(regex);
    console.log(`Testing: "${str}"`);
    
    if (matches) {
      console.log(`  ✓ Matches: ${matches}`);
      if (matches.length > 1) {
        console.log(`  Groups: ${matches.slice(1)}`);
      }
    } else {
      console.log('  ✗ No match');
    }
  });
}

// Example usage
const emailPattern = '^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$';
const testEmails = [
  'user@example.com',
  'invalid-email',
  'test.email+tag@domain.co.uk'
];

testRegex(emailPattern, testEmails);

Python Testing:

import re

def test_regex(pattern, test_strings, flags=0):
    """Test regex pattern against multiple strings"""
    compiled_pattern = re.compile(pattern, flags)
    
    for test_str in test_strings:
        match = compiled_pattern.search(test_str)
        print(f'Testing: "{test_str}"')
        
        if match:
            print(f'  ✓ Match: {match.group()}')
            if match.groups():
                print(f'  Groups: {match.groups()}')
            print(f'  Position: {match.start()}-{match.end()}')
        else:
            print('  ✗ No match')
        print()

# Example usage
email_pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$'
test_emails = [
    'user@example.com',
    'invalid-email',
    'test.email+tag@domain.co.uk'
]

test_regex(email_pattern, test_emails)

Common Use Cases

Form validation pattern development
Data extraction and parsing
Input sanitization testing
Log file analysis patterns
Search and replace validation
API response parsing
Configuration file processing
Text processing automation