Ok, I don't know regex, not mod rewrite... I'm doing something I think mod_rewrite can handle, but I don't have alot of time and the docs aren't sinking in.
I want this in an htaccess file in the root of the website.
When a user visits
http://www.mysite.com/bob/ (which bob doesn't exist)(make sure this is with our without trailing slash)
they get http://www.mysite.com/index.php?aff=bob
If http://www.mysite.com/bob/ DOES exist, they get it.