These are latin characters. I used them on IPB too when users registere with Arabic characters, which I did not want to happen.
If you use these, they should only be able to use a-z, A-Z, 0-9 space, so what is mentioned in there.
Everything which is out of that definition can not be used.
So if you want -_ space and dot, this should do the trick for you.
this will also prevent them from registering with [ and/or ] characters and : or other characters can't be used either.
You can also check this link which I got from Brogan when I had this question. It's to test regexp and see what it does.