A hexadecimal digit will be in one of these ranges: A-F, a-f, or 0-9.
#include <ctype.h> #include <stdio.h> int main(void) { char ch; for(;;) { ch = getchar(); if(ch == '.'){ break; } if(isxdigit(ch))(){ printf("%c is hexadecimal digit\n", ch); } } return 0; }