unpack are functions within perl that can do some data transformations.
The innermost part of this is
(b7)* - and you are correct that this is a bit string. However, it is only taking 7 bits at a time:
the b and B formats pack a string that's that many bits long. Each such format generates 1 bit of the result. These are typically followed by a repeat count like
You are taking a string that is 8 bits long, chops off the high bit and rewriting it back out as 8 bit data chunks.
Starting from the beginning of the input string, each 8-tuple of characters is converted to 1 character of output. With format b , the first character of the 8-tuple determines the least-significant bit of a character; with format B , it determines the most-significant bit of a character.
If the length of the input string is not evenly divisible by 8, the remainder is packed as if the input string were padded by null characters at the end. Similarly during unpacking, "extra" bits are ignored.
For this, you would be taking something that is probably ASCII or similar 7 bit data that always has its high bit as 0, stripping off the leftmost bit (see below), and rewriting it out as a constant stream of data (rather than every 8th bit being 0).
The key to this is that it reads in 8 bits at a time with the
b and only uses 7 of them.
Of note, there's a little bit of additional 'huh' going on in this code from how one typically looks at data and numbers. The
b is reading the data little endian. This means that the leftmost bit is the one that is stripped off.
Lets take the string
az and unpack it with
b7. The result is
1000011 0101111 (I put a space there so that I could more easily read it and separate the bits).
'a' in binary is
01100001 and 'z' is
01111010 as big indian. Note the leftmost bit is
unpack('(b7)*','az') will read it little endian (backwards from what one typically think) and drop the least significant (little endian) bit which is the leftmost one.
Now, that the always 0 leftmost bit of some (probably) ascii is gone, the entire thing is rewritten back out.
! show equivalent spots between the lines)