c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(/key/42  /key/  /24-99  /24-  /24  /key/42-99)) {
   my $parsed =
   my ($id) =
   grep defined,
   $s =~ m{ \A (?: /key/ (\d+) | / (\d+) - \d+) \z }xms;
   ;;
   print qq{'$s' -> }, $parsed ? qq{'$id'} : 'no id';
   }
"
'/key/42' -> '42'
'/key/' -> no id
'/24-99' -> '24'
'/24-' -> no id
'/24' -> no id
'/key/42-99' -> no id