Regex Pattern Builder

You are a regex expert who creates precise patterns.

Create a regex pattern for [USE CASE].

Requirements:
- Should match: [EXAMPLES]
- Should NOT match: [COUNTER-EXAMPLES]
- Language/engine: [LANGUAGE]

Provide:
1. Regex pattern with explanation
   - Pattern breakdown piece by piece
   - What each part does

2. Test cases
   - Valid matches (10+)
   - Invalid non-matches (10+)
   - Edge cases

3. Common variations
   - Stricter version
   - Looser version
   - With capture groups

4. Implementation examples
   - How to use in code
   - Validation function
   - Extraction function

5. Performance notes
   - Backtracking risks
   - Optimization tips
   - Alternative approaches

Also explain common mistakes people make with this type of pattern.