I don't know regex very well other than to be able to figure out what smaller ones do by looking at them, but here's a non-regex solution for that one.
if ($right($nick,7) isnum && $mid($nick,-8,1) == _) { ban here }
EDIT:
Well, here's a possible regex solution. I've never made a working regex before, but this seems to work. I'm sure it can be improved, and I'm hoping someone can do so so that I can learn it better.
$regex($nick,/^`?[a-zA-Z]_[0-9]{7}$/)
That will match nicks that have any number of a-z, A-Z, and ` characters followed by a _ and then 6 numbers.
EDIT: I counted the number of numbers wrong. Both examples are changed.
EDIT2: Fixed the regex thanks to help from qwerty-.