OK, you asked for a regex. Here it is. I think it’s immediately obvious why it’s not a good idea to validate a date with a regular expression:
First, the verbose, commented version to at least make understanding this beast possible:
^ # start of string
(?: # either match...
(?:
(?: # 31st day of all allowed months
(?:(?:0?[13578]|1[02])/31)
| # or
(?:(?:0?[13-9]|1[0-2])/(?:29|30))
) # 29th/30th day of any month except February
/ # plus any year since 1600
(?:1[6-9]|[2-9]\d)
\d{2}
)
| # or
(?: # match Feb 29th
0?2/29/
(?: # in all leap years since 1600
(?:
(?: # century
1[6-9]|[2-9]\d
)
(?: # two-digit years divisible by four, not ending in 00
0[48]
|
[2468][048]
|
[13579][26]
)
|
(?: # all the leap years ending in 00
(?:16|[2468][048]|[3579][26])
00
)
)
)
)
| # or
(?: # (for any month)
(?:0?[1-9])
|
(?:1[0-2])
)
/
(?: # match the 1st-28th day
0?[1-9]|1\d|2[0-8]
)
/
(?:
(?:1[6-9]|[2-9]\d)\d{2}
)
)$
Or, if you can’t use verbose regexes in ASP.NET validators:
^(?:^(?:(?:(?:(?:(?:0?[13578]|1[02])/31)|(?:(?:0?[13-9]|1[0-2])/(?:29|30)))/(?:1[6-9]|[2-9]\d)\d{2})|(?:0?2/29/(?:(?:(?:1[6-9]|[2-9]\d)(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))|(?:(?:0?[1-9])|(?:1[0-2]))/(?:0?[1-9]|1\d|2[0-8])/(?:(?:1[6-9]|[2-9]\d)\d{2}))$)$
These allow but do not require a leading zero in single-digit months/days. If you don’t want that, replace all instances of 0? with 0.