my $output = '';

while ($input =~ /(.)/sg) {
   my $ch = $1;
   my $ord = ord($ch);

   $output .= $ch
      if $ord >= 0xD800 && $ord <= 0xDFFF
      || $ord >= 0xFDD0 && $ord <= 0xFDEF
      || ($ord & 0xFFFF) == 0xFFFE
      || ($ord & 0xFFFF) == 0xFFFF
      || $ord >= 0x110000;
}