Aiuto regular expression

[whisher]

$str= "Mon Oct  6 13:41:45 CEST 2008: vl-commcli(2716): executing command \"vmstat | tail -1 | head -1 | awk '{ print $13 \" \" $14 \" \" $15 }'";
$num= 0;
$chunk1= '';
$chunk2= '';
for($i=0;$i<strlen($str);$i++) {
    if($str[$i]==':'){
        $num++;
    }
    if($num < 3){
        $chunk1.= $str[$i];
    }
    else{
         $chunk2.= $str[$i];
    }
}
var_dump($chunk1);
echo "
";
var_dump($chunk2);