rot13 implementations, first on the shell and then in Perl

rot13-shell-function

calid@xen ~
[✔]▶ function rot13 { echo $1 | tr 'A-Za-z' 'N-ZA-Mn-za-m'; }

calid@xen ~
[✔]▶ rot13 'Fraq hf gur pbqr lbh hfrq gb qrpbqr guvf zrffntr'
Send us the code you used to decode this message

rot13.pl

#!/usr/bin/env perl

use strict;
use warnings;

my $UPPER_MIN = ord('A');
my $UPPER_MAX = ord('Z');

my $LOWER_MIN = ord('a');
my $LOWER_MAX = ord('z');

sub valid_uppercase {
    my ($letter) = @_;

    return ord($letter) >= $UPPER_MIN && ord($letter) <= $UPPER_MAX;
}

sub valid_lowercase {
    my ($letter) = @_;

    return ord($letter) >= $LOWER_MIN && ord($letter) <= $LOWER_MAX;
}

sub rot13_letter {
    my ($letter) = @_;

    my $r13;

    if ( valid_uppercase($letter) ) {
        $r13 = ord($letter) + 13;

        if ($r13 > $UPPER_MAX) {
            # wrap back to start of uppercase range
            $r13 = $UPPER_MIN + $r13 % ($UPPER_MAX + 1);
        }
    }

    if ( valid_lowercase($letter) ) {
        $r13 = ord($letter) + 13;

        if ($r13 > $LOWER_MAX) {
            # wrap back to start of lowercase range
            $r13 = $LOWER_MIN + $r13 % ($LOWER_MAX + 1);
        }
    }

    if (!$r13) {
        die "Invalid letter '$letter'";
    }

    return chr($r13);
}

sub rot13_word {
    my ($word) = @_;

    return join '', map { rot13_letter($_) } split '', $word;
}

# make sure we really do have the list of words and not just quoted args,
# e.g. handle ./rot13.pl 'foo bar' 'baz quux'
my $args_string = join  ' ', @ARGV;
my @words       = split ' ', $args_string;

print join ' ', map { rot13_word($_) } @words;

Try this...

#!/bin/perl -p
tr/A-Za-z/N-ZA-Mn-za-m/

or as a one liner

perl -pe 'tr/A-Za-z/N-ZA-Mn-za-m/'

Of course these take a file or a pipeline, rather than converting an argument!