Author: edward[at]debian.org
[Added to the original trac issue at 3.22pm, Friday, 30th March 2007]

curl http://www.freethepostcode.org/currentlist|perl -ne'/ ([A-Z0-9]+) (\d)[A-Z0-9]+/and$p{$1}{$2}=1}for$o(sort keys%p){@i=grep!$p{$o}{$_},1..9;@i and print"$o needs inner parts for ",join(", ",map"$o $_",@i)," but they may not exist\n"'

Author: edward[at]debian.org
[Added to the original trac issue at 3.33pm, Friday, 30th March 2007]

or slightly less cryptically:

use strict;
use warnings;

my %postcode;
/ ([A-Z0-9]+) (\d)[A-Z0-9]+/ and $postcode{$1}{$2} = 1 while <>;
foreach my $outer (sort keys %postcode) {
    my inner = grep !$postcode{$outer}{$_}, 1..9 or next;
    print "$outer needs inner parts for ",
        join(", ", map "$outer $_", inner),
        " but they may not exist\n"
}