I did something like that the other day. It was at home, so I don't have the exact code handy, but basically I did opendir, read into an array, grep on the fnames of interest, sort them, pull off the last (highest) one, and parse out the digits. It's a good one-liner (not including the open/close). It's indeed much simpler to use fixed number of digits. if you put the digits last you can use the magical increment operator on the whole string.

—John