Код:

<script type="text/javascript">
function select_text(elem) {
	if(window.getSelection) {   
var s=window.getSelection();   
if(s.setBaseAndExtent){   
	s.setBaseAndExtent(elem,0,elem,elem.innerText.length-1);   
}
else {   
	var r=document.createRange();   
	r.selectNodeContents(elem);   
	s.removeAllRanges();   
	s.addRange(r);
}   
	}
	else if(document.getSelection){   
var s=document.getSelection();   
var r=document.createRange();   
r.selectNodeContents(elem);   
s.removeAllRanges();   
s.addRange(r);   
	}
	else if(document.selection){   
var r=document.body.createTextRange();   
r.moveToElementText(elem);   
r.select();
	}   
}
</script>
<script type="text/javascript">
var div = document.getElementById('pun-main').getElementsByTagName('div');
for(x in div){
	if(div[x].className=='code-box'){
div[x].getElementsByTagName('strong')[0].innerHTML = '<a href="#" onclick="select_text(this.parentNode.parentNode.childNodes[1].getElementsByTagName(\'pre\')[0]); return false;">Выделить код</a>';
	}
}
</script>