Perfetto...
a chi servisse (oppure per me se lo perdessi) ehehehe

codice:
<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">

<meta name="GENERATOR" content="Microsoft FrontPage 4.0">

<meta name="ProgId" content="FrontPage.Editor.Document">

<title>Nuova pagina 1</title>

<script>
function controlla(obj) {
a = new Array(7,13,4,9) 
t = a[obj.Tipo.selectedIndex];
	n = obj.valore.value;

	RE = /\D/ 
	if (RE.test(n)) {
		alert("solo numeri nel codice!")
		return false
	}
	if (n.length!=t) {
		alert("il codice deve essere di "+t+" cifre!")
		return false
	}
	return true
}
</script>
</head>
<body>
<form method="POST" action="form.php" onsubmit="return controlla(this)">



<select size="1" name="Tipo">
 <option>Tipo A</option>
 <option>Tipo B</option>
 <option>Tipo C</option>
</select>
<input type="text" name="valore" size="20">
<input type="submit" value="Invia" name="B1">
<input type="reset" value="Reimposta" name="B2">
</p>
</form>


</body>

</html>