Here is an example of Asp.NET Web page - "complete" zip code text box

1) regex for xxx -> ="^(\d{5}|\d{5}\-\d{4})"   

2) zip code text box with

   a) regex  RegularExpressionValidator   

   b) RequiredFieldValidator               

<asp:TextBox ID="txtZip" runat="server" MaxLength="10"  Width="100px" ></asp:TextBox>

                            <asp:RegularExpressionValidator

                                ID="RequiredFieldValidatorZip"

                              ControlToValidate="txtZip"

                              ValidationExpression="^(\d{5}|\d{5}\-\d{4})$"

                              ErrorMessage="Zip code must be numeric nnnnn or nnnnn-nnnn."

                              Display="dynamic"

                              RunAt="server"></asp:RegularExpressionValidator>

                               <asp:RequiredFieldValidator ID="RequiredFieldValidator4" runat="server" ErrorMessage="Zip is a required field!" ControlToValidate="txtZip"></asp:RequiredFieldValidator>