Regex Builder
FlaggedWrites, explains, and debugs regular expressions across languages with test cases and step-by-step breakdowns.
$ Copy the SKILL.md file to your project's .claude/skills/ directory About This Skill
Regex Builder is the most practical utility skill for any developer who works with text processing, data validation, or string manipulation. It goes far beyond generating a pattern — it teaches you why the pattern works.
How It Works
Pattern Generation Describe what you want to match in plain English: - "Match US phone numbers in formats like (555) 123-4567 or 555-123-4567" - "Extract all URLs from HTML but ignore relative paths" - "Validate ISO 8601 dates but reject dates with months > 12"
The skill generates the pattern with explicit named capture groups where applicable.
Step-by-Step Explanation Every generated pattern comes with a token-by-token breakdown: ``` ^ Start of string (\+?1)? Optional US country code (+1 or 1) [\s-.]? Optional separator \(? Optional opening parenthesis ([0-9]{3}) Area code — 3 digits \)? Optional closing parenthesis ... ```
Test Case Generation Produces matching and non-matching test cases covering edge cases, Unicode inputs, and common failure modes.
Language Flavors Adapts syntax for Python `re`/`regex`, JavaScript `/pattern/flags`, Go `regexp`, PHP PCRE, and more. Highlights differences (lookbehind support, Unicode property escapes, etc.) between flavors.
Use Cases
- Writing regex patterns for data validation (email, phone, URL formats)
- Explaining and debugging complex inherited regex patterns
- Converting regex patterns between Python, JavaScript, and PCRE flavors
- Generating comprehensive test cases for regex validation
Pros & Cons
Pros
- +Explains every pattern token-by-token, not just outputs the result
- +Generates edge case test suites automatically
- +Supports multiple language flavors with difference highlights
- +Works for both writing new patterns and debugging existing ones
Cons
- -Very complex patterns (e.g., full email RFC 5321 compliance) may still need manual review
- -Cannot execute regex against live data — testing must be done externally
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