Your code works on my machine. What problems are you finding?

update: oops, the position of the 'MAC ...'

I wouldn't bother using substr() in the sort. It doesn't provide any benefit, only takes up time, and the sort comparison function has to traverse the string whether comparign the strings or doign the substring. Now if you were doing that oncec, at the begining of the sort ( Schwartzian Transform ), it might be worth it, but that depends on how long your list is.