It uses a regular expression, so you can get pretty complicated with it.

If you want it to show anything that's a drake OR undead, you should be able to do something like drake|undead


If you want it to only show things that meet both criteria, then it's a little more involved, I think, depending on exactly what you're trying to match. I suggest looking at the filter files for examples. Or if you want to post specifics, I'm sure I or someone else could help.