I'm designing an extremely difficult challenge that combines multiple complex pattern types including:
1. Nested structures requiring balanced matching
2. Items with confusing similarities but subtle differences
3. Patterns requiring negative lookahead/lookbehind assertions
4. Items with special characters needing precise escaping
5. Red herrings that appear valid but shouldn't match
6. Mixed formats with inconsistent separators
7. Complex validation requirements (like checksums in credit cards)... (truncated)