The iswdigit function is the wide-character equivalent of the
isdigit function. It tests whether wc is a wide character
belonging to the wide character class "digit".
The wide character class "digit" is a subclass of the wide character class
"xdigit", and therefore also a subclass of the wide character class "alnum", of
the wide character class "graph" and of the wide character class "print".
Being a subclass of the wide character class "print", the wide character class
"digit" is disjoint from the wide character class "cntrl".
Being a subclass of the wide character class "graph", the wide character class
"digit" is disjoint from the wide character class "space" and its subclass
"blank".
Being a subclass of the wide character class "alnum", the wide character class
"digit" is disjoint from the wide character class "punct".
The wide character class "digit" is disjoint from the wide character class
"alpha" and therefore also disjoint from its subclasses "lower", "upper".
The wide character class "digit" always contains exactly the digits '0' to '9'.