Regex Visual Debugger
Interactive regex testing, explanation, and debugging tool.
When to Use This Skill
Activate when the user:
- Provides a regex pattern to debug
- Asks "why isn't my regex working?"
- Needs a regex pattern explained
- Wants to test regex against strings
- Asks to convert regex between flavors (Python, JS, etc.)
- Needs regex pattern suggestions
- Mentions regular expressions or pattern matching
Instructions
- Analyze the Regex Pattern
- Parse the regex structure - Identify each component (groups, quantifiers, character classes) - Check for common syntax errors - Validate regex syntax for specified flavor
- Provide Plain English Explanation
- Break down pattern piece by piece - Explain what each part matches - Describe overall pattern behavior - Clarify quantifier greediness - Explain capture groups vs. non-capturing groups
- Visual Breakdown
- Show pattern structure hierarchically - Highlight groups and alternations - Indicate character classes and ranges - Mark anchors and boundaries
- Test Against Examples
- Test provided test strings - Show what matches and what doesn't - Highlight matched portions - Explain why matches succeed or fail - Show capture group contents
- Identify Common Issues
- Unescaped special characters - Incorrect quantifiers - Greedy vs. non-greedy issues - Anchor misplacement - Unclosed groups - Flavor-specific incompatibilities
- Generate Test Cases
- Create strings that should match - Create strings that should NOT match - Include edge cases - Test boundary conditions
- Suggest Improvements
- More efficient patterns - More readable alternatives - Performance optimizations - Edge case handling
- Convert Between Flavors
- Python (re module) - JavaScript - Perl - Java - .NET - PHP (PCRE)
Output Format
# Regex Analysis: `pattern`
## Plain English Explanation
This pattern matches [description]:
- `^` - Start of string
- `[A-Z]` - One uppercase letter
- `\d{3}` - Exactly 3 digits
- `$` - End of string
**Overall**: Matches strings like "A123", "Z999"
## Visual Structure^ - Start anchor [A-Z] - Character class (uppercase letters) \d{3} - Digit, exactly 3 times $ - End anchor
## Test Results
### ✅ Matches
- `A123` → ✓ Full match
- `Z999` → ✓ Full match
### ❌ No Match
- `a123` → ✗ Lowercase 'a' doesn't match [A-Z]
- `A12` → ✗ Only 2 digits (needs 3)
- `A1234` → ✗ Too many digits
## Capture Groups
1. Group 1: `[captured text]`
2. Group 2: `[captured text]`
## Issues Found
⚠️ **Issue 1**: Pattern is too restrictive
- **Problem**: Doesn't handle lowercase letters
- **Fix**: Use `[A-Za-z]` instead of `[A-Z]`
## Suggested ImprovementsMore flexible version
^[A-Za-z]\d{3,5}$
**Changes**:
- Added lowercase letters
- Changed `{3}` to `{3,5}` for 3-5 digits
## Generated Test Cases
### Should Match
A123 Z999 B456
### Should NOT Match
1ABC (starts with digit) ABCD (no digits) A12 (too few digits)
## Flavor-Specific Notes
**JavaScript**: [any JS-specific notes] **Python**: [any Python-specific notes]
## Conversion to Other Flavors
### JavaScript
const pattern = /^[A-Z]\d{3}$/; const match = str.match(pattern);
### Python
import re pattern = r'^[A-Z]\d{3}$' match = re.match(pattern, string)
## Performance Notes
- Current complexity: O(n)
- No backtracking issues
- Consider using non-capturing groups: `(?:...)` for better performance
Examples
User: "Why doesn't this regex match emails: \w+@\w+\.\w+?" Response: Analyze pattern → Explain it matches simple emails only → Show test cases (fails on "user+tag@domain.co.uk") → Identify issues (doesn't handle special chars, multiple TLDs) → Provide improved pattern → Generate comprehensive test cases
User: "Explain this regex: ^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$" Response: Break down pattern (IP address matcher) → Explain each component → Show it matches valid IPs (0-255 per octet) → Provide test cases → Visualize structure
User: "Convert this Python regex to JavaScript: (?P<name>\w+)" Response: Identify named capture group (Python feature) → Convert to JS equivalent → Explain differences → Show both versions with usage examples
Best Practices
- Always test regex with edge cases
- Explain in plain English first
- Show concrete examples (not just theory)
- Highlight common pitfalls for each pattern
- Provide both positive and negative test cases
- Consider performance implications
- Note flavor-specific features
- Suggest simpler alternatives when possible
- Use non-capturing groups for performance
- Escape special characters properly
- Be explicit about case sensitivity
- Test with Unicode characters if relevant