The wctype function converts a valid character class defined for
    the current locale to an object of type wctype_t. The following
    character class names are defined for all locales:

    alnum        cntrl        lower        space
    alpha        digit        print        upper
    blank        graph        punct        xdigit

    Additional character class names may also be defined in the LC_
    CTYPE category of the current locale.

    See also iswctype.