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[ATCG]+  # one or more of any of A,T,C,G
         # followed by
...
\.+      # one or more dots
         # followed by
$        # the end of the string

perl -Mstrict -wE '
my $str = "ATCGATCG...";
...
'
G
7

perl -Mstrict -wE '
my $str = "ATCGATCGA...";
...
'
A
8