A regexp for date validation

dateCheck.rb

=begin rdoc
Checks that the supplied date has the correct date format '#YYYY:MM:DD' with the separators both be either - or : or /.
A year cannot start from the zero, while a month and a day can.
Doesn't check that the supplied day is within the specified month's day range. It may be further improved.
=end
def checkDate(aDate)
  %r{
    (?<year>
      (^            #no input before the start
        [1-9]\d{3}  #a year cannot start from the zero
      ) 
    )
    (?<sep>-|:|/)   #a separator can be either - or : or / 
    (?<month>       
      (0[1-9])      #a month can start from the zero
      |
      (1[0-2])      #or it could be just 10, 11 or 12 of course
    )
    \k<sep>         #the 2nd separator must be the same as the 1st met
    (?<day>
      (0[1-9])      #a day can start from the zero
      |(1[0-9])     #or should fall into the range [1..31]
      |(2[0-9])
      |30
      |31
    )$              #marks there should be the end of the input
  }x =~ aDate
end

date  = "1234:12:31"
print "date is #{checkDate(date) ? "" : "not "}correct"