need help with regex

This will do the trick:

^(?=.*[A-Z!@#$%^&*()])(?=.*[a-z])(?=.*\d).*{12,15}$

It's a series of positive lookaheads, which all verify a match without consuming any characters. Then the length validation consumes the entire string, if it's the correct length.

Match capital or special (this depends on your definition of "special"):

(?=.*[A-Z!@#$%^&*()])

Match lowercase letter:

(?=.*[a-z])

Match number:

(?=.*\d)

Verify length (this is where you would also restrict any invalid characters):

.*{12,15}

Use this Regular expression :

<asp:RegularExpressionValidator ID="rev" runat="server" ControlToValidate="txt"
           ErrorMessage="Message"
           ValidationExpression="(?=^.{8,}$)(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?!.*\s)[0-9a-zA-Z!@#$%^&*()]*$"
            ValidationGroup="validation">*</asp:RegularExpressionValidator>

Don't use a regex - it's not a "text matching" task.
Instead use code - by all means use a bunch of different regexes (or even a single regex to group the characters into their types) but check number and so forth in code - it's easier to wrote, and a lot easier to understand and maintain.

A regex to do that would be horribly difficult to understand - unlike the equivalent code.