-3-9 in the original example was -3 to +9, so it came back true when it should have been false.
It does seem that negatives aren't treated correctly with isnum. It may have to do with the fact that - isn't really negative and in this case, it is treated as a range character, so messes it up the same way a comma can mess things up.
I can't seem to find a way to make it work properly.
