This regex validates dates in YYYY-MM-DD format with comprehensive rules:
Year: Must be greater than 1900 (1901-9999)
Month: Valid range 01-12
Day: Validates correct number of days per month:
31 days: January, March, May, July, August, October, December
30 days: April, June, September, November
28 days: February (non-leap years)
29 days: February 29th only in leap years
The regex captures year, month, and day in separate groups (positions vary based on which alternative matches).