Keywords: special characters

Just wanted to ask which characters execept for numbers and letters work? I've seen apostrophs ( ' ), but nothing else yet.
I saw that the dash is already used for combining keywords, but what about the dot?
I would like to be able to use the dot ( . ) to create keywords containing more than one word, because I run in trouble using quotes in my image managment program.

At the moment the dot is removed creating an unuable keyword in terms of search engines. :rolleyes

Thanks,
Sebastian