I'm creating an extremely difficult challenge that combines multiple advanced regex concepts:
1. Complex nested structures with balanced delimiters requiring proper escape handling
2. Tricky date/time formats with optional components and varying separators
3. Advanced email patterns with quoted local parts containing special characters
4. ISBN/product codes with specific validation patterns
5. Nested XML-like tags with attributes
6. Similar but non-matching patterns to trap naive approaches
7. U... (truncated)