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:RegularExpressionVa... ID="RequiredFieldValidatorZip" ControlToValidate="txtZip" ValidationExpression="^(\d{... ErrorMessage="Zip code must be numeric nnnnn or nnnnn-nnnn." ......