$ perl -Mstrict -Mwarnings -E '
my $x = q{ac-t-c-t-g};
$x =~ /[acgt]/g for (1..4);
say substr($x, 0, pos($x));
'
ac-t-c
$ perl -Mstrict -Mwarnings -E '
my $x = q{a----c-tg-ggg};
$x =~ /[acgt]/g for (1..4);
say substr($x, 0, pos($x));
'
a----c-tg