C solution (Marius Gavrilescu)

We can iterate over each character in the format string. If s[i] is %, we call scanf with s[i]s[i+1] and the next offset. Otherwise, we call scanf with s[i].

    1 #include<stdio.h>
    2 #include<stdlib.h>
    3 #include<string.h>
    4 
    5 char s[205], f[3];
    6 unsigned char mem[128];
    7 
    8 int main(void){
    9     fgets(s, sizeof s, stdin);
   10     strtok(s, ",");
   11 
   12     for(int i = 0; s[i] ; i++)
   13         if((*f = s[i]) == '%')
   14             f[1] = s[++i], scanf(f, mem + atoi(strtok(NULL, ",")));
   15         else
   16             f[1] = 0, scanf(f);
   17 
   18     for(int i = 0 ; i < 128 ; i++)
   19         printf("%02X", mem[i]);
   20 }