The regular expression:

(?-imsx:[1-31]-[1-12]-[1-99])

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  [1-31]                   any character of: '1' to '3', '1'
----------------------------------------------------------------------
  -                        '-'
----------------------------------------------------------------------
  [1-12]                   any character of: '1' to '1', '2'
----------------------------------------------------------------------
  -                        '-'
----------------------------------------------------------------------
  [1-99]                   any character of: '1' to '9', '9'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------