# $pre[n] matches any character that ends with exactly n zero bits
my @pre = map { sprintf "[\\x%02x-\\x%02x]", 1<<(7-$_), (1<<(8-$_))-1 } 0 .. 7;
$pre[0] = "(?:^|$pre[0])";

# $suf[m] matches any character that begins with at least m zero bits
my @suf = ('', #m=0 -> match anything
           map { my $m=$_; 
                 '[' . join('', map { sprintf "\\x%02x", $_<<$m } 
                                    0..((1<<(8-$m))-1))
                     . ']'
                } 1..7);

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