codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>set selection in textarea</title>
<meta http-equiv="content-type" content="text/html; charset=utf-8" /> 
<style type="text/css">

</style>
<script type="text/javascript">
/*<![CDATA[*/
function selectRange(fieldID,start,end){
	// decommenta per estremo start compreso
	// start--;
	var tx=document.getElementById(fieldID);
	if(tx.setSelectionRange){
		tx.focus();
		tx.setSelectionRange(start,end);
	}
	else if (tx.createTextRange) {
		var range=tx.createTextRange();
		range.collapse(true);
		range.moveEnd('character',end);
		range.moveStart('character',start);
		range.select();
	}
}
/*]]>*/
</script>
</head>
<body>
<form>
<textarea id="txtarea" rows="5" cols="31">
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Morbi nonummy viverra tellus. Donec aliquam sem quis lectus. Vivamus quam lectus, interdum vel, consequat suscipit, varius in, sem. Ut nibh enim, facilisis id, tempor nec, imperdiet id, leo.
</textarea>

<input type="button" value="seleziona da 5 a 15" onclick="selectRange('txtarea',5,15)" />
<input type="button" value="seleziona da 25 a 45 " onclick="selectRange('txtarea',25,45)" />
</form>
</body>
</html>