Regular Expression to match valid dates

Question

I\'m trying to write a regular expression that validates a date. The regex needs to match the following

• M/D/YYYY
• MM/DD/YYYY
• Single digit mon

Answer 1

Perl expanded version

Note use of /x modifier.

/^(
      (
        ( # 31 day months
            (0[13578])
          | ([13578])
          | (1[02])
        )
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
          | (3[01])
        )
      )
    | (
        ( # 30 day months
            (0[469])
          | ([469])
          | (11)
        )
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
          | (30)
        )
      )
    | ( # 29 day month (Feb)
        (2|02)
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
        )
      )
    )
    [\/]
    # year
    \d{4}$
  
  | ^\d{4}$ # year only
/x

Original

^((((0[13578])|([13578])|(1[02]))[\/](([1-9])|([0-2][0-9])|(3[01])))|(((0[469])|([469])|(11))[\/](([1-9])|([0-2][0-9])|(30)))|((2|02)[\/](([1-9])|([0-2][0-9]))))[\/]\d{4}$|^\d{4}$