CODEWITHSUNDEEP
perl
jonasv
jonasv

Reputation: 75

Transliteration

I got got such line:

print "a b c" =~ tr/a-z/c-za-b/r . "\n";

Which basically translates a b c to c d e,but I don't completely understand is how this works.

So, lets have a look at this part:

tr/a-z/c-za-b/r

tr is used in perl for transliterating, which is clear and ok for me.

in the part /a-z/ part we select all lowercase letters from a to z, the second part /c-za-b/ is mystery to me, I tried to experiment but can't figure out how this works. Can someone explain how this being converted or link me to some good explanation or manual ?

Thanks

Upvotes: 1

Views: 514

Answers (2)

Jim Driscoll
Jim Driscoll

Reputation: 904

You're not alone in finding it incomprehensible. tr// is rarely used in modern code.

Anyway, in Perl-speak, if you have a line like:

$x=~tr/0-9a-f/a-p/;

That is roughly:

my @in = (0 .. 9, "a" .. "f");
my @out = ("a" .. "p");
for my $i (0 .. $#in) {
    $x =~ s/$in[$i]/$out[$i]/g;
}

It's a little more complicated than that because tr// doesn't work iteratively, but this should give you a good sense of what's going on.

Upvotes: 1

mob
mob

Reputation: 118665

a-z is expanded to the 26 character string abcdefghijklmnopqrstuvwxyz.

c-za-b is expanded to the 26 character string cdefghijklmnopqrstuvwxyzab.

Given the two strings

1. abcdefghijklmnopqrstuvwxyz
2. cdefghijklmnopqrstuvwxyzab

$string =~ tr/a-z/c-za-b/ converts each character it can find in the first row into the corresponding character in the second row.

Similar notation is used to implement the reversible rot13 encoding:

($encoded = $original) =~ tr/a-zA-Z/n-za-mN-ZA-M/;
($encoded2 = $encoded) =~ tr/a-zA-Z/n-za-mN-ZA-M/; # now $encoded2 eq $original

Upvotes: 8

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