codice:
...
<script language="JavaScript">
<!--
function controlla() {

  var val = test.prova.value;
  var len = test.prova.length;
  if (isNaN(val) || parseInt(val) > 12 || len < 2) {
    alert("errore");
    return false;
  }
  else{
  return true;
  }
}
-->
</script>
</head>
<body>
<form onSubmit="return controlla();" name="test">
...
ciao