<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
    <script type='text/javascript'>
        function OnRequestStart(ajaxPanel, eventArgs) {
            var eventTarget = eventArgs.get_eventTarget();
            if (eventTarget == "<%= Button1.UniqueID %>") {
                if (!CheckZipCode()) {
                    eventArgs.set_enableAjax(false); // cancel the ajax request
                }
            }
        }

        function CheckZipCode() {
            var zipCode = $get('<%= TextBox1.ClientID %>').value;
            if (zipCode.length != 5) {
                alert('Please enter a valid 5 digit postal code!');
                return false;
            }
            else {
                var fiveDigitCheckRE = /^\d{5}$/ //regular expression for checking a 5 digit number
                if (zipCode.search(fiveDigitCheckRE) == -1) {
                    alert("Only digits are allowed!");
                    return false;
                }
            }
            return true;
        }
    </script>
</telerik:RadCodeBlock>