<script language="javascript">
// lista dei caratteri consentiti
chrOk = "0123456789,."
function CheckL(obj) {
o = obj.value;
a = o.charAt(o.length-1);
if (chrOk.indexOf(a)==-1) {
alert("carattere non consentito");
obj.value=o.substr(0,o.length-1);
}
}
</script>
<form action="#">
<input type=text name=pippo onkeyup="CheckL(this)">