AABAABAA
AABAABAA
BBABBABB
AABAABAA
AABAABAA
BBABBABB
AABAABAA
AABAABAA

##</code><code>##

use strict;
use warnings;

#
# Two types of operation:
#
# - Pick a symbol:
#   '<ABC>' =~ /<[A-C]*([A-C])[A-C]*>/
#
# - Constraint check:
#   '<AB,AC,BA,BC,CA,CB,AAB,AAC,BBA,BCC,CCA,CCB>' =~ /<(?:[A-C]*,)*\x\y\z?(?:,[A-C]*)*>/
#


sub allowed {
   my ($syms) = @_;
   my @combos;
   for    my $y (@$syms) {
      for my $x (@$syms) {
         next if $x eq $y; 
         push @combos, "$y$x";
         push @combos, "$y$y$x";
      }
   }
   return @combos;
}


{
   my $solve       = shift || "count_solutions";  # first_solution, all_solutions, count_solutions
   my $grid_size   = shift || 8;
   my $num_symbols = shift || 6;

   my @syms = ('A'..'Z')[0..$num_symbols-1];

   my $syms_list  = join('', reverse(@syms));
   my $syms_class = "[" . $syms[0] . "-" . $syms[-1] . "]";

   my $pick_str = "<$syms_list>";
   my $pick_pat = "<$syms_class*($syms_class)$syms_class*>";

   my $constr_str = "<" . join(',',allowed(\@syms)) . ">";
   my $constr_pat_gen = sub { "<(?:$syms_class*,)*\\$_[0]\\$_[1]\\$_[2]?(?:,$syms_class*)*>" };

   my $str = '';
   my $pat = '';

   for    my $y (0..$grid_size-1) {
      for my $x (0..$grid_size-1) {
         $str .= $pick_str;
         $pat .= $pick_pat;

         my $z = $y * $grid_size + $x + 1;

         if ($x >= 2) {
            $str .= $constr_str;
            $pat .= $constr_pat_gen->($z, $z-1, $z-2);
         }

         if ($y >= 2) {
            $str .= $constr_str;
            $pat .= $constr_pat_gen->($z, $z-1*$grid_size, $z-2*$grid_size);
         }
      }
   }

   #print("$str\n");
   #print("$pat\n");

   if ($solve eq "first_solution") {
      my (@sol) = $str =~ /^$pat\z/
         or die("No solution");

      for    my $y (0..$grid_size-1) {
         for my $x (0..$grid_size-1) {
            print($sol[$y * $grid_size + $x]);
         }
         print("\n");
      }
   }
   elsif ($solve eq "all_solutions") {
      use re 'eval';
      local our $count = 0;
      $str =~ /^$pat\z(?{
         ++$count;
         for    my $y (0..$grid_size-1) {
            for my $x (0..$grid_size-1) {
               no strict 'refs';
               print(${ $y * $grid_size + $x + 1 });
            }
            print("\n");
         }
         print("\n");
      })(?!)/;
      print("$count solutions\n");
   }
   elsif ($solve eq "count_solutions") {
      use re 'eval';
      local our $count = 0;
      $str =~ /^$pat\z(?{ ++$count; print "$count\n" if $count % 10000 == 0; })(?!)/;
      print("$count solutions\n");
   }
   else {
      die("Bad value for \$solve\n");
   }
}

##</code><code>##

   F
   FF