Restricting Special Characters in Textbox

<script type="text/javascript">

        function isNumeric(keyCode)
         {
            if (keyCode == 16)
                isShift = true;

            return ((keyCode >= 48 && keyCode <= 57 || keyCode == 8 ||
            (keyCode >= 96 && keyCode <= 105) || (keyCode >= 65 && keyCode <= 90)) && isShift == false);
        }
        var isShift = false;
        function keyUP(keyCode)
         {
            if (keyCode == 16)
                isShift = false;
        }

           
</script>


<asp:TextBox id="txtUserName"  onkeyup = "keyUP(event.keyCode)" onkeydown = "return isNumeric(event.keyCode);" onpaste = "return false;" OnTextChanged="txtUserName_TextChanged"></asp:TextBox>