isxdigit —
hexadecimal-digit character test
Standard C Library (libc, -lc)
#include <ctype.h>
int
  
  isxdigit(int
    c);
The isxdigit() function tests for any hexadecimal-digit
  character.
The isxdigit() function returns zero if the character
  tests false and returns non-zero if the character tests true.
ctype(3),
  isalnum(3),
  isalpha(3),
  isascii(3),
  isblank(3),
  iscntrl(3),
  isdigit(3),
  isgraph(3),
  islower(3),
  isprint(3),
  ispunct(3),
  isspace(3),
  isupper(3),
  stdio(3),
  toascii(3),
  tolower(3),
  toupper(3),
  ascii(7)
The isxdigit() function conforms to
  ANSI X3.159-1989 (“ANSI C89”).
The argument to isxdigit() must be
  EOF or representable as an unsigned
  char; otherwise, the behavior is undefined. See the
  CAVEATS section of
  ctype(3) for more details.