Regular Expressions in Filters not working

If I create a simple filter such as *.ext - the filter works fine but if I try a regular expression like .*.ext$ the filter doesn't work. What am I doing wrong?

Also, is it possible to exclude all file extensions except the ones that I specifically nominate i.e. can I use a not (!) in a regular expression? I would like to do something like !.*.(ext1|ext2)$ to ignore all files except .ext1 and .ext2

Great program BTW.

Cheers.