Regular expression for ISO 8601 datestamps

Original.txt

^
   (\+|-)? # Extended year?
   (\d{4,}) # Year
   (
      (
         -? # Optional separator
         (0[1-9]|1[012]) # Month, 01 to 12
         (?(?<=-\d\d)-) # Require another separator if we used one before
         (0[1-9]|[12][0-9]|3[01]) # Day, 01 to 31
      )|(
         -? # Optional separator
         (00[1-9]|0[1-9]\d|[12]\d\d|3[0-5]\d|36[0-6]) # Ordinal day, 001 to 366
      )
   )
   -(.*) # Required separator and slug
   (\.[^.]+) # Required file extension
$

Ruby.txt

# Ruby does not do the conditional capturing group.

^
   (\+|-)? # Extended year?
   (\d{4,}) # Year
   (
      (
         (
            -(0[1-9]|1[012])- # Month, 01 to 12, with separators
            |
            (0[1-9]|1[012]) # Month, 01 to 12, without separators
         )
         (0[1-9]|[12][0-9]|3[01]) # Day, 01 to 31
      )|(
         -? # Optional separator
         (00[1-9]|0[1-9]\d|[12]\d\d|3[0-5]\d|36[0-6]) # Ordinal day, 001 to 366
      )
   )
   -(.*) # Required separator and slug
   (\.[^.]+) # Required file extension
$