I'm creating an exceptionally difficult challenge that combines:
1. Complex nested XML-style tags with attributes requiring proper balancing and escaping
2. ISBN numbers with variable formats and check digits
3. IPv6 addresses with different compression patterns
4. Time zone specifications with optional minutes
5. Base64 encoded strings with specific content patterns
6. Negative patterns where similar but slightly different items should be excluded
7. Mixed date/time formats with fractional seco... (truncated)