#include <ctype.h>
int isxdigit(int c);
isxdigit returns nonzero (true) if and only if the value of the argument c is a hexadecimal digit 0 through 9, a through f, or A through F.
isxdigit conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).