void replacestr(char *arg1, char *arg2, char *arg3)
{
    string output;
    char *ptr = strstr(arg1, arg2);

    if(!ptr) { result(arg1); return; } // Return the string untouched if arg2 cannot be found in arg1

    strncpy(output, arg1, ptr - arg1); // Copy every char in the original string, before the ptr, to string "output"
    output[ptr - arg1] = 0;
    sprintf(output + (ptr - arg1), "%s%s", arg3, ptr + strlen(arg2)); // Concatenate the new string to the output
    result(output);                                                   // then the remainder of the original string
}
COMMAND(replacestr, ARG_3STR);