Regular expression for date....

Question

i have a textbox
one calender control
on selecting date from calender control date goes in format "Aug 19, 2011" ("MMM dd, yyyy") into textbox
i want the regular expression that if user input date directly into textbox than regular expression is in format Aug 19, 2011 ("MMM dd, yyyy") or not.

actualy i wanted that there should be no option for manual entry into the textbox, but when i use readonly or enabled property and try to insert value into database then null value is retrieved from textbox

nyway, the regular expression will do...
can nyone help