Regular expression to match only the first file in a RAR file set

Question

To see what file to invoke the unrar command on, one needs to determine which file is the first in the file set.

Here are some sample file names, of which - naturall

Answer 1

Personally I wouldn't use (extended) regular expressions in this case (or at least not just one to do it all). What's wrong with coding this in, for example, a few ifs?