For your own sake, please learn the difference between directory and directory/ and directory/file. directory forces Apache to look for a filename called directory and, upon not finding one, then looks for directory/, the URI for the directory. Obviously directory/file is the file within the directory.
What’s this have to do with mod_rewrite and regex? mod_rewrite looks at the {REQUEST_URI} string and attempts to match that (with the regex of the RewriteRule or the regex of a RewriteCond when %{REQUEST_URI} is specified). When you add or remove that trailing slash, you’re changing the directory level of the URI and THAT will cause you a problem (because you are forced to use absolute redirections).
My solution to this is to use link to directories (directory/) when I want that directory’s DirectoryIndex to be served or to a path/file when I want that file to be served. If anyone wants to “play games” on my sites, I can’t necessarily stop them from using “stupid links” but I can prevent them from getting anything more than a 404 response (which is more than they deserve).
That makes my response: Don’t bother with it, it’s not worth the effort.