asp.net - Masked Edit Extender not working with Reg Expression Validator -

i have following control masked edit phone number. have tested regular expression , appears working correctly. however, when have attempted enter correct phone number, returns error message , not sure why.

 <asp:textbox id="txtphone" width="10em" runat="server" maxlength="14"    validationgroup="test"></asp:textbox>   <asp:regularexpressionvalidator validationexpression="((\(\d{3}\) ?)| (\d{3}-))?\d{3}-\d{4}"   controltovalidate="txtphone" id="regexpval_txtphone" runat="server"   cssclass="field-validation-error" errormessage="invalid format (use: (xxx)   xxx-xxxx)" display="dynamic" forecolor="red" validationgroup="test"  />            <ajaxtoolkit:maskededitextender id="txtphone_maskededitextender"   runat="server" mask="(999) 999-9999" targetcontrolid="txtphone"   clearmaskonlostfocus="false" /> 

any ideas why this should not work?