When the user passes in a charset currently, it's only used for dot. I'm expanding this to be use intersection between the charset passed in, and categories like \w\s\d as well, but don't intend to for literals.

Should it apply to character classes? I'm not sure.

For some, like [^\w] it's pretty clear it should (once Unicode support lands), but others like [a-z_] are already fairly limited.