[ATCG]+  # one or more of any of A,T,C,G
         # followed by
([ATCG]) # exactly one of A,T,C,G
         # which is captured into $1
         # followed by
\.+      # one or more dots
         # followed by
$        # the end of the string

##</code><code>##

perl -Mstrict -wE '
my $str = "ATCGATCG...";
if($str=~/[ATCG]+([ATCG])\.+$/) {
   my $last_char=$1;
   say $last_char;
    my $pos = rindex($str, $last_char);
   say $pos;
}
'
G
7

##</code><code>##

perl -Mstrict -wE '
my $str = "ATCGATCGA...";
if($str=~/[ATCG]+([ATCG])\.+$/) {
   my $last_char=$1;
   say $last_char;
    my $pos = rindex($str, $last_char);
   say $pos;
}
'
A
8