Regex Pattern Expert

## Curation Note Regular expressions remain one of the most powerful yet frustrating tools in programming. This skill emerged from extensive community discussions where developers struggle to write, read, and debug regex patterns. The approach emphasizes explanation alongside patterns because regex without context is nearly unmaintainable. Claude's ability to break down complex patterns step-by-step has made this one of the most appreciated AI use cases. ## Pattern Creation Framework ### Step 1: Define Requirements Before writing regex, clarify: - What should match? - What should NOT match? - What are the edge cases? - What regex flavor? (JavaScript, Python, PCRE, etc.) ### Step 2: Build Incrementally ```regex \S+@\S+ [a-zA-Z0-9._%+-]+@\S+ [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ``` ### Step 3: Explain the Pattern Always break down patterns: ``` ^ # Start of string [a-zA-Z0-9._%+-]+ # Local part: letters, numbers, or ._%+- @ # Literal @ symbol [a-zA-Z0-9.-]+ # Domain name: letters, numbers, dots, hyphens \. # Literal dot [a-zA-Z]{2,} # TLD: at least 2 letters $ # End of string ``` ## Common Patterns Library ### Email Validation ```regex ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ``` ### URL Matching ```regex https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*) ``` ### Phone Numbers (US) ```regex ^(\+1)?[-.\s]?\(?[0-9]{3}\)?[-.\s]?[0-9]{3}[-.\s]?[0-9]{4}$ ``` ### Date Formats ```regex ^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$ ^(0[1-9]|1[0-2])\/(0[1-9]|[12]\d|3[01])\/\d{4}$ ``` ### Password Strength ```regex ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$ ``` ### IP Addresses ```regex ^((25[0-5]|(2[0-4]|1\d|[1-9]|)\d)\.?\b){4}$ ``` ## Flavor Differences | Feature | JavaScript | Python | PCRE | | ------------ | -------------- | --------------- | -------------- | | Named groups | `(?<name>...)` | `(?P<name>...)` | `(?<name>...)` | | Lookbehind | Limited | Yes | Yes | | Unicode | `/u` flag | Default | `(*UTF8)` | | Multiline | `/m` flag | `re.MULTILINE` | `(?m)` | ## Performance Considerations ```regex ^(a+)+$ ^(a++)$ ^(?>a+)$ .*foo.*bar.* [^f]*foo[^b]*bar.* ``` ## Testing Patterns Always test with: 1. Valid inputs that should match 2. Invalid inputs that should NOT match 3. Edge cases (empty, very long, special chars) 4. Real-world data samples ## Best Practices 1. **Keep it readable** - Use comments and whitespace (x flag) 2. **Be specific** - `\d{3}` over `.+` 3. **Anchor when possible** - Use `^` and `$` 4. **Avoid catastrophic backtracking** - Test with long inputs 5. **Document patterns** - Future you will thank you