my @allowed = qw[ AA AG GC GT CA CG TT TC ];
my $allowed = join "|", @allowed;
my $regex   = qr/ N+ | (?: (?=$allowed) . )*  . /x;

$data="TGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGATAG
C";
print "$_\n" for $data =~ m/$regex/g;

##</code><code>##

G
C