SSN Validation

 Hi ..
I want to validate ssn textbox. i am validated through regular expression  validator.
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="ssntxt" ErrorMessage="Enter Valid SSN Number 'xxx-xx-xxxx'" ValidationExpression="\d{3}-\d{2}-\d{4}" Display="Dynamic" SetFocusOnError="True"></asp:RegularExpressionValidator>
I want to do somemore changes in my ssntextbox.when the user enters 3 numbers i want to add automatically enter into my textbox  '-' (this symbol) then user able to enter 2 numbers then  automatically enter into '-' (this symblo)then user able to add 4 numbers.
is there any other way to implement to enter ssn number.
How can i do this.?? i am learning .net technologies. could any one suggest me how to procceed? ...