>perl -wMstrict -le
"my $class = qr{ [actg] }xms;
 ;;
 for my $str (qw(ac-t-c-t-g  a----c-tg-ggg  --a-c-t-g-a-a-a--)) {
   $str =~ m{ \A ((?: -* $class){4}) }xms;
   print qq{from '$str' -> ($1)};
   }
"
from 'ac-t-c-t-g' -> (ac-t-c)
from 'a----c-tg-ggg' -> (a----c-tg)
from '--a-c-t-g-a-a-a--' -> (--a-c-t-g)