<script language="javascript" type="text/javascript">
function isNumberKey(evt)
{
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
{
if (charCode != 46)
{
alert("only Numbers are allowed");
return false;
}
}
return true;
}
</script>
txt_postal_code.Attributes["onkeypress"] = "javascript:return isNumberKey(event);";
Or use Compare Validator
<asp:TextBox ID="txt_postal_code" runat="server">
<asp:CompareValidator ID="intValidator" runat="server" ControlToValidate="txt_postal_code" Operator="DataTypeCheck" Type="Integer" ErrorMessage="Value must be int">