Perl: Given an arbitrary string, how do you extract the first N bits?

Given a string $old_str, I am trying to extract the first N bits (not bytes) into $new_str. I have been reading the pack documentation and perlpacktut, but am hopelessly confused. This is where I currently stand:

my $old_str = "9876543210";                                                     
# Extract the first 5 bits
my $new_str = pack("B5", unpack("B*", $old_str));
printf "%#b | %#b\n", $new_str, $old_str;

This produces:

0b1000 | 0b1001001100101100000001开发者_JAVA技巧011011101010

But I want this:

0b10010 | 0b1001001100101100000001011011101010

You want the vec built-in: vec

You can use unpack:

my $bit_string = unpack( 'b*', $old_str );

It will create a string of 80 '1's (char 0x31s) and '0's (char 0x30s). On Windows, you will probably need to add reverse.

I’m not sure what you are trying to do here. The unpack("B*", $old_str) creates a bit string containing the following bits (spaces added for readability):

00111001 00111000 00110111 00110110 00110101 00110100 00110011 00110010 00110001 00110000

…in other words, a bit string corresponding to ASCII numbers for your characters:

$ perl -E "printf('%#b ', ord) for split(//, '9876543210')"
0b111001 0b111000 0b110111 0b110110 0b110101 0b110100 0b110011 0b110010 0b110001 0b110000

Then you do pack('B5', '00111001…') which seems to be a bit complex. It looks like pack returns a byte consisting of the five rightmost bits in the first 8-tuple (11001). That gives 56 or the 8 string (since the ASCII for 8 is 56):

$ perl -E "say ord pack('B5', '00111001…')"
56
$ perl -E "say pack('B5', '00111001…')"
8

And when you printf the string, you get the binary numeric representation of the number 8:

$ perl -E "say printf '%#b', '8'"
0b10001

(This is crazy.)