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>