You can use the "cut" operator in order to prevent backtracking. That way you can stay closer to your original code.

   /\d{4}-(?>\d{1,3})(?!-)/
[download]

But, you still might want to include that digit in the negative lookahead, or you still can get unexpected matches.

for (qw(2005-12 2005-100 2005-1- 2005-12- 2005-123- 2005-1000)) {
    printf "%-10s: ", $_;
    if ( /\d{4}-(?>\d{1,3})(?!-)/ ) {
      print "yep\n" ;
    } else {
      print "nope\n";
    }
}
[download]

Result:

2005-12   : yep
2005-100  : yep
2005-1-   : nope
2005-12-  : nope
2005-123- : nope
2005-1000 : yep

So, better make it

   /\d{4}-(?>\d{1,3})(?![\-\d])/
[download]

In this case, the cut operator becomes close to useless. Well, it doesn't hurt.

Update I shouldn't post in a hurry. I now see ikegami has posted a post very similar to mine. Duh.