ISBN.pl

#!/usr/bin/env perl

use strict;
use warnings;

print 'ISBN: ';
$_ = readline(*STDIN);

if (/\d{10}/) {
  s/^\s+|\s+$//g;
  my $sum = 0;
  for (my $i = 1; $i < length($_); $i++) {
    $sum += $i * (substr($_, $i-1, 1) + 0);
  }
  if (4 == $sum % 11) {
    print "ok\n";
    exit;
  }
}
print 'ng';
print "\n";