/(\b
  (([a-z])                        # at least one char
   ?{local @suffix; push @suffix, $^N}) # build up suffix
  (([a-z)
   ?{local @suffix; push @suffix, $^N}
   | \S+)+
  [a-z]?                             # possible odd char
  (([a-z])
   (?{if ($^N eq $suffix[-1]) {pop @suffix}})
   | \S+)+)
  (([a-z])
   (?{if ($^N eq $suffix[-1]) {pop @suffix}})
 \b)
/x