validation expression for gmail,yahoo

<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtEmail"Display="Dynamic" ErrorMessage="Invalid Email" ValidationExpression="^([a-zA-Z0-9]+[a-zA-Z0-9._%-]*@(yahoo\.com|gmail\.com))"                                            ValidationGroup="reg"></asp:RegularExpressionValidator>