# $pre[n] matches any character that ends with exactly n zero bits
@pre = map { qr{[@{[chr(1<<(7-$_))]}-@{[chr((1<<(8-$_))-1)]}]} } 0 .. 7;
$pre[0] = qr(^|$pre[0]);

# $suf[m] matches if we have at least m zero bits following
my @suf = ('', #done
           map { my $m=$_;
                 qr{[@{[join '', map { chr($_<<$m) }
                                 0..((1<<(8-$m))-1)]}]} }
               1..7
           );

sub zero_bit_regexp {
  my $N = shift;
  my $p = join '|', map { qr{$pre[$_]\0{@{[($N-$_)>>3]}}$suf[($N-$_)&0x7]} } 0..7;
  return qr{$p};
}