I'm using regex in a htaccess file to catch /[username] and re-direct this to a username page, for example, mywebsite.com/john will forward to mywebsite.com/username?u=john. This works, but I now want to exclude anything that matches a directory which exists. So, if someone goes to mywebsite.com/sign-up, this is EXCLUDED for the re-direction, because that directory (/sign-up) exists. Is that possible, and is it normal practice? Thank you.
Edit: Ideally I don't want to have to exclude every existing directory manually. I'd like it to be automatic for all existing directories.