Select Code to Download

([AGCT])(?!\1)([AGCT])(?!\1|\2)([AGCT])(?!\1|\2|\3)[AGCT],([AGCT])(?!\
+4)([AGCT])(?!\4|\5)([AGCT])(?!\4|\5|\6)[AGCT]

my $str = 'some_str';

...
}

print $str;