$str='...............................GAGAACATTAGTGGGTGCAGCGCACAAGCATGGCACATGTATACGTATGTAA..................';

##</code><code>##

$rightmost_position_of_letter = rindex ($str,[A|T|C|G]);

##</code><code>##

$rightmost_position_of_letter = rindex ($str,[ATCG]);