TextBox on failing Custom (Client) Validation Still causes Postback

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ASP.NET

Hi,

I have a textbox who has CausesValidation set to True, TextChange event enabled with corresponding eventhandler in the ServerSide Code, and also a CustomValidator code in client Side JavaScript

<asp:textbox id="txtCustZipcode" size="10" tabindex="9" runat="server"
    MaxLength="10" AutoPostBack="true" OnTextChanged="txtCustZipcode_TextChanged"
    CausesValidation="True"></asp:textbox>
<asp:CustomValidator ID="cstValidate_txtCustZipCode" runat="server"
    ClientValidationFunction="fnzipcodeValidation" ControlToValidate="txtCustZipcode"
    SetFocusOnError="True"></asp:CustomValidator>

function fnzipcodeValidation(source, args)
{
    var strZipCodeValidator = /[&"#+>]/;
    args.IsValid=true;
    if(document.forms[0].txtCustZipcode.value!='')
        {
            if (strZipCodeValidator.test(trim(document.forms[0].txtCustZipcode.value)))
                {
                    alert('ZipCode cannot take &,#,+,> and double quotes, Pls correct');
                    document.forms[0].txtCustZipcode.focus()
                    document.forms[0].txtCustZipcode.select()
                    args.IsValid=false;
                    return false;
                }
        }
}

The thing is, even if the args.IsValid=false, the Server Side TextChanged event occurs.

Okay, I tried this on the Server Side EventHandler:

Protected Sub txtCustZipcode_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtCustZipcode.TextChanged
       cstValidate_txtCustZipCode.Validate()
  If cstValidate_txtCustZipCode.IsValid Then
'Do Something
  End If
End Sub

This works fine, but the trouble with this approach is: did you notice the alert message on the fnZipCodeValidation??? That fires almost four times!

Any clever, simple solution to over come this???