>perl -wMstrict -le
"my $alt = qr{ s[ ]m[ ]e[ ]f | s[ ]f[ ]pl | s[ ]f }xms;
 my $rex = qr{ (?<= .) $alt (?= .)                }xms;
 ;;
 for my $s (
   'x  s m e f  s f pl  s f  x',  'x  s f  s f pl  s m e f  x',
   'x  s f  s m e f  s f pl  x',  'x  s f  s f  x',
   'x s s x',  's f',  's m e f',
   ) {
   local our $longest;
   local our $offset;
   use re 'eval';
   $s =~ m{
     ($rex)
     (?{ ($longest, $offset) = ($1, $-[1])
       if ! defined($longest) || length($1) > length($longest)
       })
     (*SKIP)(*FAIL)
     }xmsg;
   print qq{'$s' -> '$longest' at $offset} if defined $longest;
   }
"
'x  s m e f  s f pl  s f  x' -> 's m e f' at 3
'x  s f  s f pl  s m e f  x' -> 's m e f' at 16
'x  s f  s m e f  s f pl  x' -> 's m e f' at 8
'x  s f  s f  x' -> 's f' at 3