Sorry. OK, so if you walk the ltmpoolmemberstat mib for example, you get (sample output):
F5-BIGIP-LOCAL-MIB::ltmPoolMemberStatServerBytesIn."int-pool".ipv4."10.10.1.10".443
F5-BIGIP-LOCAL-MIB::ltmPoolMemberStatServerBytesIn."int-pool".ipv4."10.18.1.11".443
Notice that the pool definition, which is what cacti would use as the index for each pool member, is in the middle of the string, not at the end, as in the virtual server definition. The numerical representation is below, with the index highlighted.
.1.3.6.1.4.1.3375.2.2.5.4.3.1.6.11
.105.110.116.45.112.111.111.108.1.4.10.1.1.10.443
.1.3.6.1.4.1.3375.2.2.5.4.3.1.6.11
.105.110.116.45.112.111.111.108.1.4.10.1.1.11.443
My experimentation with the regex in the xml file always led to a double dot in the oid string used to query the LTM, which obiously failed.
I'm not sure how to approach from the script side. I think it would be ideal to figure out a way to have Cacti "discover" the pools and pool members from an iControl script, then use iControl to populate the rrd files as you define what to graph in the device page. I think a thread on Cacti forum would be the best bet for this help, and we can reach out to Joe here at DevCentral on the iControl specific pieces.