Add some more comments

Signed-off-by: Timur Tabi <timur@tabi.org>
Don't add any characters that we don't recognize

The else-clause added any unrecognized characters to the list as-is.
Instead, we want to ignore and drop those characters.

Signed-off-by: Timur Tabi <timur@tabi.org>
Convert all characters to lowercase

Signed-off-by: Timur Tabi <timur@tabi.org>