my $str  = join '|',
  sort { length $b <=> length $a || $a cmp $b } keys %tokens_to_match;
$string =~
  s/(\b|\s|\.|\[|\])($str)(?>(\b|\s|\.|\[|\]))/
    $1 . ( exists($tokens_to_match{lc $2}) 
      ? $tokens_to_match{lc $2} 
      : $2
    )
  /gei;