Replace junk charater function

public string cleanip(string str)
    {
        string str_op;
        str_op = str;
        if (str_op != "" && str_op != null)
        {
            str_op = str_op.Replace("<", "");
            str_op = str_op.Replace(">", "");
            str_op = str_op.Replace("&lt;", "");
            str_op = str_op.Replace("&gt;", "");
        }
        if (str_op != null)
        {
            if (str_op.Length >= 50)
                str_op = str_op.Remove(50);
        }
        return str_op;
    }

validation with ajax filter

  
 <asp:TextBox ID="txtMessage" runat="server" TextMode="MultiLine" autocomplete="off"  TabIndex="15"></asp:TextBox>
                                    </div>
                                    <div class="errormsg_contact">
                                        <asp:RequiredFieldValidator ID="rfvYourMessage" runat="server" ControlToValidate="txtMessage"
                                Display="Dynamic" ErrorMessage="Please enter your message" ValidationGroup="SUBMIT"
                                SetFocusOnError="true"> </asp:RequiredFieldValidator>

                                <cc1:FilteredTextBoxExtender ID="FilteredTextBoxExtender3" runat="server"
                                FilterType="Custom, Numbers, UppercaseLetters, LowercaseLetters" ValidChars=" ,./\()_" TargetControlID="txtMessage"> </cc1:FilteredTextBoxExtender>

                                        <asp:RegularExpressionValidator ID="rvtxtMessage" runat="server" ControlToValidate="txtMessage"
                                Display="Dynamic" ErrorMessage="Message contains minuman 5 maximum 1000 characters with or without space"
                                ValidationExpression="[,./\()_-a-zA-z0-9 ]{5,1000}" ValidationGroup="SUBMIT"
                                SetFocusOnError="true" />

validation more than zero

 function fnChkNumVal(source, args) {
            //var iknum = parseInt(args.Value)
            var iknum =args.Value
            if(iknum==0)
                 args.IsValid = false;
            else
                 args.IsValid = true ; 
        }

 <asp:TextBox ID="txtNWOnlineTransAmt" MaxLength="16" runat="server" CssClass="input">

  <asp:CustomValidator ID="CustomValidator19" runat="server" ErrorMessage="Transaction amount can not be zero"
                                                    EnableClientScript="true" SetFocusOnError="true" ClientValidationFunction="fnChkNumVal"
                                                    ControlToValidate="txtNWOnlineTransAmt" ValidationGroup="Feedback" Display="None"
                                                    CssClass="commment"></asp:CustomValidator>