Custom Field Validation Controls

I am having some difficulties with some validation controls on a custom form. What is wrong with this code?

<div class="CommonFormFieldName">
   <CSControl:ResourceControl ID="ResourceControl4" runat="server" ResourceName="CreateNewAccount_ZipCode" />
           </div>
               <div class="CommonFormField">
          <asp:TextBox id="ZipCode" runat="server" MaxLength="128" columns="40" />
           <asp:RegularExpressionValidator runat="server" ID="RegularExpressionValidator1"
                                         ControlToValidate="ZipCode" EnableClientScript="false"
                                         ErrorMessage="Zip code must be of the form 11111-1111"
                                         InitialValue="" width="100%" cssclass="validationWarning"
                                         ValidationExpression="\d{5}(-\d{4})?">**
                                </asp:RegularExpressionValidator>
               </div>

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