Regular Expression Validaor Control Won't work on Teleriik RadMaskedTextBox

So I have my regular expression now, and it works just fine on a regular TextBox control, but it won't work on the Telerik RadMaskedTextBox.  I don't know much about Telerik controls.  This is my first time working with them.  Can someone tell me how to get the validation to work on this control?  
<telerik:RadMaskedTextBox ID="txtFederalTaxID" runat="server" Mask="##-#######" CssClass="field" SelectionOnFocus="SelectAll"> </telerik:RadMaskedTextBox> <asp:RequiredFieldValidator ID="RequiredFieldValidator28" runat="server" ErrorMessage="*" ControlToValidate="txtFederalTaxID" InitialValue="" ValidationGroup="Terms"></asp:RequiredFieldVa ...