$string="ctaacxxxxxxxxxxxxxxxxxxxxxxx";
$_=$string;
$_=/(ct(a|g)a(c|t))(.{20,50})$/i;
print "\n$1,$2,$3,$4";
($branch,$pyrich)=($1,$4);
print "\n$branch$pyrich";

##</code><code>##

ctaac,a,c,xxxxxxxxxxxxxxxxxxxxxxx
ctaacxxxxxxxxxxxxxxxxxxxxxxx