# From
$str1 = "ATC[TG]CC";

# into 
$ans_of_str1 = [    #
                'ATCTCC',
                'ATCGCC'
               ];

##</code><code>##

# From
$str2 = "ATCGC[ATCG]";

# into 
$ans_of_str2 = [      #
                'ATCGCA',
                'ATCGCT',
                'ATCGCC',
                'ATCGCG',
               ];

##</code><code>##

# From
$str3 = "ATCGCT";

# into 
$ans_of_str3 = [ 'ATCGCT' ];