I can't answer any of those questions regarding the extra question. I'm unfamiliar with using that method to stop bot registrations since I use other methods. As for regular expressions, right under that input field title, it says:
You may require the input field to match a regular expression (
PCRE).
Click on the link and look at the examples and you'll learn a bit about them. Or google it. I am not good at writing them, so you'll have to have someone else help with that.