in reply to Re: Re: Simple Encryption question
in thread Simple Encryption question

If your file is small, just read it into an array first: 

open F, $file or die $!;
my @data = <F>;
for my $i ( 0..$#data ) {
    if ( $i == 0 or $i == $#data ) {
        #first or last line so do whatever
        print $data[$i];
    }
    else {
        # not first or last line
        $data[$i] =~ s#^(\d{16})#$_ = $1; tr/0-9/A-J/; $_#e
        print $data[$i];
    }
}

[download]

cheers

tachyon

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Re: Re: Re: Re: Simple Encryption question
by rman (Initiate) on Dec 30, 2003 at 20:26 UTC
    Brilliant! This works fine.

    I am curious to know how this statement works:

    $data[$i] =~ s#^(\d{16})#$_ = $1; tr/0-9/A-J/; $_#e

    I understand some pieces of it:
    -> the s is for substitution; ^d16 is for first 16digits;
    -> tr does the conversion. # delimits;

    Can you please let me know, how the whole piece fits in to do the substitution.

    thanks
[reply]
[d/l]
      s/this/that/
s#this#that#
s#this# "th" . "at" #e   <-- the /e means (e)valuate the RHS before do
+ing substitution

# so the LHS of our RE is just this
^        begin string
(        begin capture into $1
\d{16}   16 digits
)        end capture into $1


# when we get a match of 16 digits at the begining of the string then 
+we have 2 events. 
# first $1 contains them and 
# second the RHS of the RE gets evaluated.

$_ = 1;      # set a var to $1 so we can modify it (can't do to $1 as 
+read only)
tr/0-9/A-J/; # transliterate contents of $_ aka $1
$_;          # perl will eval this with the net result that 
             # our 'that' result is $_ which duly gets used
             # to replace our original digits

      [download]

      /e is quite handy at times. The reason we need the naked $_ at the end of the RHS is because when perl evaluates a function (like the RHS) the return value is the last thing Perl evaluated. Without the $_ this would be the return value from tr which is not the transliterated string itself but rather the integer count of the number of transliterations. So we put the $_ there. You could put "$_ hello" or anything else you liked and that is what would get subbed in.

      cheers

      tachyon

[reply]
[d/l]
        Thanks for the patient explanation. I get it now. Also, A quick one hour reading of this doc http://www.perldoc.com/perl5.8.0/pod/perlretut.html helped me remember the fundamentals of regex cheers!.
[reply]

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