#A(?:(?:(?:(?:0?[13578])|(1[02]))/31/(19|20)?dd)|(?:(?:(?:0?[13-9])|(?:1[0-2]))/(?:29|30)/(?:19|20)?dd)|(?:0?2/29/(?:19|20)(?:(?:[02468][048])|(?:[13579][26])))|(?:(?:(?:0?[1-9])|(?:1[0-2]))/(?:(?:0?[1-9])|(?:1d)|(?:2[0-8]))/(?:19|20)?dd))Z#

A           # start of string
 (?:         # group without capture
             # that match 31st of month 1,3,5,7,8,10,12
   (?:       # group without capture
     (?:     # group without capture
       (?:   # group without capture
         0?  # number 0 optionnal
         [13578] # one digit either 1,3,5,7 or 8
       )     # end group
       |     # alternative
       (1[02]) # 1 followed by 0 or 2
     )       # end group
     /       # slash
     31      # number 31
     /       # slash
     (19|20)? #numbers 19 or 20 optionnal
     dd    # 2 digits from 00 to 99
   )         # end group
|
   (?:(?:(?:0?[13-9])|(?:1[0-2]))/(?:29|30)/(?:19|20)?dd)
|
   (?:0?2/29/(?:19|20)(?:(?:[02468][048])|(?:[13579][26])))
|
   (?:(?:(?:0?[1-9])|(?:1[0-2]))/(?:(?:0?[1-9])|(?:1d)|(?:2[0-8]))/(?:19|20)?dd)
 )
Z

require 'date'
def valid_date?( str, format="%m/%d/%Y" )
  Date.strptime(str,format) rescue false
end

[0-1][0-9]/[0-3][0-9]/[0-9]{2}(?:[0-9]{2})?

(?:(09|04|06|11)(/|-|.)(0[1-9]|[12]d|30)(/|-|.)((?:19|20)dd))|(?:(01|03|05|07|08|10|12)(/|-|.)(0[1-9]|[12]d|3[01])(/|-|.)((?:19|20)dd))|(?:02(/|-|.)(?:(?:(0[1-9]|1d|2[0-8])(/|-|.)((?:19|20)dd))|(?:(29)(/|-|.)((?:(?:19|20)(?:04|08|12|16|20|24|28|32|36|40|44|48|52|56|60|64|68|72|76|80|84|88|92|96))|2000))))

^((0?1?1){1}|(0?1?2){1}|([0]?3|4|5|6|7|8|9))/((0?1?2?3?1){1}|(0?1?2?(2|3|4|5|6|7|8|9|0))|(30))/[1-90]{4}$