I want to avoid user to press " and ' characters.

What must i do in keyup event?

It is better to validate the form on submission, as well
character strings can equal representations of the character you dont want, "

<script type='text/javascript'>
function isAlphabet(elem, helperMsg){
 var alphaExp = /^["']+$/;
 if(elem.value.match(alphaExp)){
  alert(helperMsg);
  elem.focus();
 return true;
}else{ return false; }
}
</script>
<form>
Letters Only: <input type='text' id='letters' onkeyup="isAlphabet(document.getElementById('letters'), 'Cannot contain quote characters')" />
</form>

Fragment only, not checked for long strings

You could also try:

<html>
<body>
<script type="text/javascript">
function notAQuote(e) {
	var keynum;
	var keychar;
	var numcheck;

	if(window.event) // IE
		keynum = e.keyCode;
	else if(e.which) // Netscape/Firefox/Opera
		keynum = e.which;

	var notQuote;

	switch (keynum) {
		case 34:
			notQuote = false;
			break;
		case 39:
			notQuote = false;
			break;
		default:
			notQuote = true;
	}

	return notQuote;
}
</script>

<form>
<input type="text" onkeypress="return notAQuote(event)" />
</form>

</body>
</html>
Comments
thank you.

You can also try this one:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">
<html id="html40L" lang="en">
<head>
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="Content-Style-Type" content="text/css">
<meta http-equiv="Content-Script-Type" content="text/javascript">
<title>X-Window</title>
<style type="text/css">
<!--
input {
   display : block;
   height : 15%;
   letter-spacing : 4px;
   margin : .300em 0 1em 0;
   width : 30%; }  
-->
</style>
<script type="text/javascript">
<!--
var quot = function( sdQuote ) {
var quoted = "";

   sdQuote = (( document.getElementById ) ? document.getElementById( sdQuote ) : document.all[ sdQuote ] );

   filterQ = sdQuote.value.match(/[\"\']+/g); 
   for ( var x = 0; x < filterQ.length; x++ ) {
      if ( filterQ[ x ] ) {
      quoted += "[ " + (( x ) + 1 ) + " " + String( filterQ[ x ] ).fontcolor("blue") + " ] ";
      sdQuote.value = sdQuote.value.replace( filterQ[ x ], "" );
      (( document.getElementById ) ? document.getElementById( "count" ) : document.all.count ).innerHTML = "Invalid Character" + (( filterQ.length > 1 ) ? "s" : "" ) + " : " + quoted;
      continue;
      }
   }
};

//-->
</script>
</head>
<body>
<div id="main">
<form id="testform" name="testform" action="*" onsubmit="return false;">
<div>
<label for="txt">Test Field: <input type="text" id="txt" name="txt" onkeyup="quot(( this.id |= 'txt' ));" value=""></label>
<div id="count"></div>
</div>
</form>
</div>
</body>
</html>
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