while (<DATA>) {
/^.*?,.*?,.*?,(.*?),/;
print "$1\n";
}
__DATA__
6,0,3,"8.1",1
7,578,,"8.2,r1",1
8,0,5,"8.2,r3",1
18,0,13,"6.2",1
19,D610,,"6.3,r1",1
20,2f78,15,"6.3,r2",1