RegularExpressionValidator for Contact No.:

<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtContactNumber"
                                ForeColor="Red" ErrorMessage="Invalid Number." ValidationExpression="^(?:\d{10},?\b)+$"
                                Display="Dynamic" ValidationGroup="AttendeeValidationGroup"></asp:RegularExpressionValidator>