<asp:RequiredFieldValidator ID="reqValidator" runat="server" 
            ErrorMessage="Enter Password" ControlToValidate="txtPassword" 
            SetFocusOnError="True">?</asp:RequiredFieldValidator>

 <asp:RegularExpressionValidator ID="regValidator" runat="server" 
            ErrorMessage="Invalid mail address" ControlToValidate="txtMail" 
            SetFocusOnError="True" 
            ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">?</asp:RegularExpressionValidator>

    <asp:RangeValidator ID="ranValidator" runat="server" 
            ErrorMessage="Invalid data" ControlToValidate="txtPassword" 
            MaximumValue="100" MinimumValue="0" SetFocusOnError="True" Type="Double">?</asp:RangeValidator>

     <asp:CompareValidator ID="cmpValidator" runat="server" 
            ErrorMessage="Retype password" ControlToCompare="txtPassword" 
            ControlToValidate="txtRePassword">?</asp:CompareValidator>