unpack

unpack TEMPLATE, EXPR

This function does the reverse of pack: it takes a string (EXPR) representing a data structure and expands it out into a list value, returning the list value. (In a scalar context, it can be used to unpack a single value.) The TEMPLATE has much the same format as in the pack function - it specifies the order and type of the values to be unpacked. (See pack for a more detailed description of TEMPLATE.)

Here's a subroutine that does (some of) substr, only slower:

sub substr {
 my($what, $where, $howmuch) = @_; if ($where < 0) {
 $where = -$where;
return unpack "\@* X$where a$howmuch", $what;
}
else {
 return unpack "x$where a$howmuch", $what;
}
}

and then there's:

sub signed_ord {
 unpack "c", shift }

Here's a complete uudecode program:

#!/usr/bin/perl $_ = <> until ($mode,$file) = /^begin\s*(\d*)\s*(\S*)/; open(OUT,"> $file") if $file ne ""; while (<>) {
 last if /^end/; next if /[a-z]/; next unless int((((ord() - 32) & 077) + 2) / 3) == int(length() / 4);
print OUT unpack "u", $_;
}
chmod oct $mode, $file;

In addition, you may prefix a field with %number to indicate that you want it to return a number-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:

undef $/; $checksum = unpack ("%32C*", <>) % 32767;

The following efficiently counts the number of set bits in a bit vector:

$setbits = unpack "%32b*", $selectmask;

Here's a simple MIME decoder:

while (<>) {
 tr#A-Za-z0-9+/##cd; # remove non-base64 chars tr#A-Za-z0-9+/# -_#; # convert to uuencoded format $len = pack("c", 32 + 0.75*length); # compute length byte print unpack("u", $len . $_); # uudecode and print }