Regex Pattern Builder

Master regular expressions instantly. Generate complex regex patterns with explanations, test cases, and practical examples. Works with any regex flavor - JavaScript, Python, PCRE, and more.

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Prompt:

You are a regex expert who can create, explain, and debug regular expressions for any use case. Make regex approachable by providing clear explanations and practical examples.

I need a regex pattern for the following:

REGEX REQUIREMENTS:
What to Match: [DESCRIBE WHAT YOU WANT TO MATCH]
What NOT to Match: [WHAT SHOULD BE EXCLUDED]
Language/Tool: [JAVASCRIPT / PYTHON / PERL / GREP / SED / ETC]
Examples of Valid Matches: [PROVIDE 3-5 EXAMPLES]
Examples of Invalid Matches: [PROVIDE 3-5 EXAMPLES]
Special Requirements: [CASE SENSITIVITY / MULTILINE / GLOBAL / ETC]

Please provide:

1. THE REGEX PATTERN
Complete pattern with explanation:
```regex
/[YOUR PATTERN HERE]/[FLAGS]
```

For different flavors:
- **JavaScript**: `/pattern/flags`
- **Python**: `r"pattern"` with re.FLAGS
- **PCRE/PHP**: `/pattern/flags`
- **Java**: `Pattern.compile("pattern", FLAGS)`
- **.NET**: `new Regex(@"pattern", Options)`

2. PATTERN BREAKDOWN
Explain each part:
```
^                    # Start of string
([A-Z])              # Group 1: Capital letter
[a-z]+               # One or more lowercase letters
(?:ing|ed|s)?        # Non-capturing group: optional suffix
\s*                  # Zero or more whitespace
(\d{2,4})            # Group 2: 2-4 digits
$                    # End of string

Full pattern: /^([A-Z])[a-z]+(?:ing|ed|s)?\s*(\d{2,4})$/
```

3. VISUAL EXPLANATION
Show how it matches:
```
Input: "Walking 2024"
       ^^^^^^^  ^^^^
       |        |
       |        └─ (\d{2,4}) captures "2024"
       └─ ([A-Z])[a-z]+(?:ing) matches "Walking"

✅ MATCH

Input: "walk 24"
       ^^^^
       |
       └─ No capital letter at start
       
❌ NO MATCH
```

4. TEST CASES
Comprehensive testing:
```javascript
// Test data with expected results
const testCases = [
    // Valid matches
    { input: "[example1]", shouldMatch: true, groups: ["group1", "group2"] },
    { input: "[example2]", shouldMatch: true, groups: ["group1", "group2"] },
    
    // Invalid matches
    { input: "[invalid1]", shouldMatch: false },
    { input: "[invalid2]", shouldMatch: false },
    
    // Edge cases
    { input: "", shouldMatch: false },
    { input: "[edge case]", shouldMatch: true/false }
];

// Test function
function testRegex(pattern, testCases) {
    testCases.forEach(test => {
        const result = pattern.test(test.input);
        const matches = test.input.match(pattern);
        console.log(`Input: "${test.input}"`);
        console.log(`Expected: ${test.shouldMatch}, Got: ${result}`);
        if (matches) console.log(`Groups: ${matches.slice(1)}`);
    });
}
```

5. COMMON VARIATIONS
Alternative patterns:
```regex
# Strict version (exact match):
/^exact-pattern$/

# Lenient version (partial match):
/pattern/

# Performance optimized:
/(?:non-capturing|groups)/

# Unicode support:
/[\u{1F600}-\u{1F64F}]/u

# Multiline mode:
/^pattern$/m
```

6. REGEX FLAVORS
How it differs across languages:

**JavaScript:**
```javascript
const pattern = /your-pattern/gi;
const matches = text.match(pattern);
const replaced = text.replace(pattern, 'replacement');
```

**Python:**
```python
import re
pattern = re.compile(r'your-pattern', re.IGNORECASE)
matches = pattern.findall(text)
replaced = pattern.sub('replacement', text)
```

**grep/sed:**
```bash
# grep with extended regex
grep -E 'pattern' file.txt

# sed substitution
sed 's/pattern/replacement/g' file.txt
```

7. PERFORMANCE OPTIMIZATION
Make it faster:
```regex
# Avoid catastrophic backtracking:
❌ Bad:  /(x+)+y/
✅ Good: /x+y/

# Use atomic groups when possible:
(?>atomic-group)

# Lazy vs Greedy:
.*?  # Lazy (matches minimum)
.*   # Greedy (matches maximum)

# Anchors improve performance:
^start  # Faster than searching entire string
```

8. DEBUGGING TIPS
Common issues and fixes:
```
Problem: Matches too much
Fix: Use lazy quantifiers (??, *?, +?)

Problem: Doesn't match multiline
Fix: Add 'm' flag or use [\s\S] instead of .

Problem: Case sensitivity
Fix: Add 'i' flag or use [Aa] for specific chars

Problem: Special characters not working
Fix: Escape them: \. \* \+ \? \[ \] \( \) \{ \} \| \\ \^ \$
```

9. REAL-WORLD EXAMPLES
Practical applications:

**Email Validation:**
```regex
/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/
```

**URL Extraction:**
```regex
/https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)/g
```

**Phone Numbers:**
```regex
/(?:\+?1[-.\s]?)?\(?([0-9]{3})\)?[-.\s]?([0-9]{3})[-.\s]?([0-9]{4})/
```

10. ONLINE TOOLS
Test and refine:
- regex101.com - Test with explanations
- regexr.com - Visual regex builder
- regexpal.com - Simple tester
- regexcrossword.com - Learn through puzzles

11. CHEAT SHEET
Quick reference:
```
ANCHORS
^     Start of string
$     End of string
\b    Word boundary
\B    Not word boundary

CHARACTER CLASSES
.     Any character except newline
\d    Digit [0-9]
\D    Not digit
\w    Word character [A-Za-z0-9_]
\W    Not word character
\s    Whitespace
\S    Not whitespace

QUANTIFIERS
*     0 or more
+     1 or more
?     0 or 1
{n}   Exactly n
{n,}  n or more
{n,m} Between n and m

GROUPS
(...)   Capturing group
(?:...) Non-capturing group
(?=...) Positive lookahead
(?!...) Negative lookahead
(?<=..) Positive lookbehind
(?<!..) Negative lookbehind
```

12. REPLACEMENT PATTERNS
Using captured groups:
```javascript
// JavaScript
text.replace(/(first)(second)/, '$2 $1');  // Swap groups

// Python
re.sub(r'(\w+) (\w+)', r'\2, \1', text)  # Last, First

// sed
sed 's/\(.*\) \(.*\)/\2 \1/' file.txt
```

Ensure the regex is:
- As simple as possible
- Well-documented
- Tested with edge cases
- Performant for large inputs
- Compatible with specified language

Pro Tips:

Provide real examples of what should and shouldn't match
Specify the programming language or tool you're using
Mention if you need Unicode or special character support
Include performance requirements for large text processing
Ask for multiple variations (strict vs lenient) if unsure

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