Update: 'Borrowed' tilly's list of states. (my result is a 113 character string - though I too am now wondering where 'KT' is ... I'm told its a brand of whiskey, so I'll keep it in - besides, it is now the 'official' list for this challenge :-)

Another update: Added dws's fix.

#!/usr/local/bin/perl -l -w

use strict;

my @states=qw(
AK AL AR AZ CA CO CT DC DE FL GA HI IA ID IL IN KS
KT KY LA MA MD ME MI MN MO MS MT NC ND NE NH NJ NM
NV NY OH OK OR PA RI SC SD TN TX UT VA VT WA WI WV
WY);

my %states;
@states{@states} = ();

my @results;
my $is_done;
while (%states) {
 my ($letter, $is_first);
 ($letter, $is_first, $is_done) = get_next_letter(\%states);
 push(@results, keys %states), last if $is_done;
 my $pos = $is_first ? 0 : 1;
 my @states = grep { substr($_,$pos,1) eq $letter } keys %states;
 my $chr_class =
  '[' . join('', map { substr($_, 1-$pos, 1) } @states) . ']';
 push @results, $is_first ? $letter.$chr_class : $chr_class.$letter;
 delete @states{@states};
}

print '/^(',join("|", @results),')$/';

sub get_next_letter {
 my $states = shift;
 my (%first, %second, $is_done);
 for (keys %$states) {
  my ($first, $second) = split '';
  $first{$first}++;
  $second{$second}++;
 }
 my ($first) = sort { $first{$b} <=> $first{$a} } keys %first;
 my ($second) = sort { $second{$b} <=> $second{$a} } keys %second;
 $is_done = 1 if $first{$first} == 1 and $second{$second} == 1;
 ($first{$first} > $second{$second}) ?
  ($first, 1, $is_done) : ($second, 0, $is_done);
}

# 113 characters!
# Add 5 characters ('pop=~' to the beginning) if you want
# it to work on @_ instead of $_
/^([WLPCVGIM]A|N[CDEHJMVY]|M[DEINOST]|[VKUC]T|A[ZKLR]|[WHR]I|O[HKR]|I[
+LND]|[SD]C|[KW]Y|T[XN]|KS|SD|DE|FL|WV|CO)$/
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