$ perl -le'"aaaaab" =~ /[aA](?{ print pos })[bB]/'
1
2
3
4
5

##</code><code>##

$ perl -le'"aaaaac" =~ /[aA](?{ print pos })[bB]/'
1
2
3
4
5

##</code><code>##

$ perl -le'"aaaaab" =~ /[aA](?{ pos })[bB](?{ print $^R })/'
5

$ perl -le'"aaaaac" =~ /[aA](?{ pos })[bB](?{ print $^R })/'

$