Hello SK,
You're best bet to accomplish a validation like this is to use a jQuery datepicker. Don't allow the user to type into the textfield, force them to select from the datepicker. From the datepicker, you can alter the jQuery file to disable / not include weekends. This way you're forcing the user to select a business day/weekday.
There's no need to try to find a Regular Expression that will validate this, chances are you won't find such an expression. You could use straight JavaScript too to determine whether the date selected from the textfield was a weekday or weekend and conditionally proceed from there.
Here's a great forum post that may assist you:
http://stackoverflow.com/questions/...end-or-not If you need assistance configuring this functionality, you can utilize Data Springs newly formed "Premium Integration Services".
Let me know if you have any questions.
Thanks,
Ryan