VMS Help CC, Run-time functions, iswxdigit  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns a nonzero integer if its wide-character argument is a
  hexadecimal digit (0 to 9, A to F, or a to f) in the program's
  current locale.  Otherwise, it returns 0.

  Syntax:

       #include <wctype.h>  (ISO C Standard)
       #include <wchar.h>   (XPG4 Standard)

       int iswxdigit (wint_t wc);