validation in c#.net

<cc1:FilteredTextBoxExtender ID="FilteredTextBoxExtender4"  runat="server" FilterType="Numbers"
                                                            TargetControlID="txtNo">
                                                        </cc1:FilteredTextBoxExtender>

<asp:TextBox id="txtPayDate" runat="server" Width="287px" MaxLength="64" BorderColor="LightGray" BorderStyle="Solid" BorderWidth="1px"></asp:TextBox>
                                                                                    
                                                                                    <asp:RequiredFieldValidator id="RequiredFieldValidator16" runat="server" ValidationGroup="AddUser" ErrorMessage="Enter Payment Date" Display="None" SetFocusOnError="True" ControlToValidate="txtPayDate"></asp:RequiredFieldValidator>
                                                                                    
                                                                                    <cc1:ValidatorCalloutExtender id="ValidatorCalloutExtender16" runat="server" TargetControlID="RequiredFieldValidator16"></cc1:ValidatorCalloutExtender>
                                                                                    <cc1:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtPayDate"
                                                            Format="dd MMM yyyy"></cc1:CalendarExtender>
                                                        <cc1:ValidatorCalloutExtender ID="ValidatorCalloutExtender17" runat="server" TargetControlID="RegularExpressionValidator1"></cc1:ValidatorCalloutExtender>
                                                        <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtPayDate"
                                                            ErrorMessage="Invalid Format" ValidationExpression="^((31(?!\ (Feb(ruary)?|Apr(il)?|June?|(Sep(?=\b|t)t?|Nov)(ember)?)))|((30|29)(?!\ Feb(ruary)?))|(29(?=\ Feb(ruary)?\ (((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13579][26])|((16|[2468][048]|[3579][26])00)))))|(0?[1-9])|1\d|2[0-8])\ (Jan(uary)?|Feb(ruary)?|Ma(r(ch)?|y)|Apr(il)?|Ju((ly?)|(ne?))|Aug(ust)?|Oct(ober)?|(Sep(?=\b|t)t?|Nov|Dec)(ember)?)\ ((1[6-9]|[2-9]\d)\d{2})$"
                                                            ValidationGroup="AddFAQ"></asp:RegularExpressionValidator> 

    <script type="text/javascript">;
        function onlyNumbers(event) {
            var e = event || evt; // for trans-browser compatibility
            var charCode = e.which || e.keyCode;
            if (charCode > 31 && (charCode < 48 || charCode > 57))
                return false;
            return true;
        }
    </script>;
//In page load of the aspx page
   txtAcStNo.AddTextBoxAttribute("onkeypress", "return onlyNumbers(event);");