sub mismatches {
  my($source, $target) = @_;
  my @sparts = ($source =~ /(\[.*?\]|.)/g);
  my @tparts = ($target =~ /(\[.*?\]|.)/g);
  scalar grep { 
    my($s, $t) = ($sparts[$_], $tparts[$_]);
    $s !~ /\[/ ? ($s !~ /$t/)
    : $t !~ /\[/ ? ($t !~ /$s/)
    : !intersect($s, $t) 
  } 0 .. $#sparts;
}

sub intersect {
  my($s, $t) = @_;
  my %seen = map +($_ => 1), $s =~ /[^\[\]]/g;
  scalar grep $seen{$_}, $t =~ /[^\[\]]/g;
}