How to validate a DATE with a regular expression?

 Never use a regular expression to validate a date. The regular expression is only useful to validate the format of the date as entered by a user. For the actual date validity, you should rely on another language.

The following expressions will validate the number of days in a month but will NOT handle leap year validation; hence february can have 29 days every year, but not more.

ISO date format (yyyy-mm-dd):

^[0-9]{4}-(((0[13578]|(10|12))-(0[1-9]|[1-2][0-9]|3[0-1]))|(02-(0[1-9]|[1-2][0-9]))|((0[469]|11)-(0[1-9]|[1-2][0-9]|30)))$

ISO date format (yyyy-mm-dd) with separators '-' or '/' or '.' or ' '. Forces usage of same separator accross date.

^[0-9]{4}([- /.])(((0[13578]|(10|12))\1(0[1-9]|[1-2][0-9]|3[0-1]))|(02\1(0[1-9]|[1-2][0-9]))|((0[469]|11)\1(0[1-9]|[1-2][0-9]|30)))$

United States date format (mm/dd/yyyy)

^(((0[13578]|(10|12))/(0[1-9]|[1-2][0-9]|3[0-1]))|(02/(0[1-9]|[1-2][0-9]))|((0[469]|11)/(0[1-9]|[1-2][0-9]|30)))/[0-9]{4}$

Hours and minutes, 24 hours format (HH:MM):

^(20|21|22|23|[01]\d|\d)((:[0-5]\d){1,2})$