If you want to be picky, I'ld go with:

    if ((x >= 'a' && x <= 'z') ||
        (x >= 'A' && x <= 'Z') ||
        (x >= '0' && x <= '9'))

As in a regular text file you're more likely to hit an alphanumerical character than a number. If the first OR statement is true, any good compiler will skip the next two clauses.