/sys$common/syshlp/helplib.hlb CC, Run-time functions, isdigit  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns a nonzero integer if its argument is classed as a digit in
  the program's current locale.  Otherwise, it returns 0.

  Syntax:

       #include <ctype.h>

       int isdigit(int character);