Prova questo
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
function testRe(aValue) {
//----- primo carattere una lettera seguita da lettere o numeri o underscore
pattern = "^[a-zA-Z][a-zA-Z0-9_]+$"
var a = new RegExp(pattern);
if (a.test(aValue)) {
alert("ok");
return true;
}
alert("! ok");
return false;
}
//-->
</script>
<input type="text" id="text"><input type="button" value="Test" onclick="testRe(document.getElementById('text').value)">
</body>
</html>