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

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

  Syntax:

       #include <ctype.h>

       int isxdigit(int character);