Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm reading the documentation of 'pack'. I cannot understand this: 
unpack("a3/A A*", "007 Bond  J ")       gives (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.")  gives ("Bond, J", ".")

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"You write "/"sequence-item and the repeat count is obtained by popping off the last element from the stack. The sequence-item must not have a repeat count. If sequence-item refers to a string type ("A", "a", or "Z"), the length-item is the string length, not the number of strings."
What does it all mean? I think when the doc talks about popping from the stack, that only applies to the numeric data. Or something. So yeah, how does that work?

Also:

"Pack and unpack can operate in two modes: character mode ("C0" mode) where the packed string is processed per character, and UTF-8 mode ("U0" mode) where the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte basis."
What's that all about? It means that with U0 pack treats the string as C-style bytearray? Or what?

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Re: Pack documentation
by choroba (Cardinal) on Jun 11, 2014 at 09:18 UTC
    a3/A takes the first three bytes and interprets them as a number (7 in this case). It then reads 7 bytes and interprets them as a space padded string (A). So, ' Bond  ' is shortened to ' Bond' only. The following A* just takes the rest as a string, removing trailing spaces. The second example is left as an exercise for the reader.

    I don't understand your third question. Could you rephrase? Have you studied the examples in the paragraph that follows your citation? 

    $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
      perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9

$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
      perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89

$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
    perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89

$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
    perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89

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Re: Pack documentation (unpack U0 C0 / repeat)
by Anonymous Monk on Jun 11, 2014 at 03:32 UTC
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Re: Pack documentation
by LanX (Saint) on Jun 11, 2014 at 02:50 UTC
    > I'm reading the documentation of pack .

    Did you already have a look into perlpacktut ?

    Cheers Rolf

    (addicted to the Perl Programming Language)

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      Yes.
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