Io continuo a ritenere che programmare senza jQuery fa guadagnare anni luce in chiarezza…:

codice:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
(function () {

	var rDeny = /"/;

	function filterInput (oEvent) {
		var nChr = (oEvent || /* IE */ window.event).charCode;
		return nChr === 0 || !rDeny.test(String.fromCharCode(nChr));
	}

	function createFilter () {
		for (var aTxtBoxes = document.getElementsByClassName("filtered"), nLen = aTxtBoxes.length, nItem = 0; nItem < nLen; nItem++) {
			aTxtBoxes[nItem].onkeypress = filterInput;
		}
	}

	window.addEventListener ? addEventListener("load", createFilter, false) : window.attachEvent ? attachEvent("onload", createFilter) : (onload = createFilter);

})();
</script>
</head>

<body>
<form name="myForm">


Esempio #1: <input type="text" name="myInput1" class="filtered" /></p>


Esempio #2: <input type="text" name="myInput2" class="filtered" /></p>


Esempio #3: <input type="text" name="myInput3" class="filtered" /></p>
</form>
</body>
</html>