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

##</code><code>##

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";
    }
}

##</code><code>##

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