sub utf8_escape_2 {
    my $s = shift;

    $s =~ s{
               (
                   [\xc2-\xdf] [\x80-\xbf]
                   |
                   [\xe0-\xef] [\x80-\xbf]{2}
                   |
                   [\xf0-\xf4] [\x80-\xbf]{3}
               )
           }
           {
               my @c = map ord, split //, $1;
               '\\u' .
               (
                  ( 2 == @c ) ?
                      sprintf( '%04x',
                               ( ( 0b00011111 & $c[0]) << 6 )
                               | ( 0b00111111 & $c[1] )
                      )
                : ( 3 == @c ) ?
                      sprintf( '%04x',
                               ( ( 0b00001111 & $c[0] ) << 12 )
                               | ( ( 0b00111111 & $c[1] ) << 6 )
                               | ( 0b00111111 & $c[2] )
                      )
                : 'fffd'
               );
           }xemsg;

    $s =~ tr/\x80-\xff//d; # invalid utf8

    return $s;
}