def count_vowels(word):
    """
    Returns the number of vowels (a, e, i, o, u) in a string using recursion

    Examples:
        >>> count_vowels('Autodifferentiation')
        10
        >>> count_vowels("avada kedavra")
        6
        >>> count_vowels("STAMPEDE")
        3
        >>> count_vowels("stpd")
        0
        >>> count_vowels('A '*350)
        350
    """
    vowel = ['A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u']
    if word == "":
        return 0
    elif word[0] in vowel:
        return count_vowels(word[1:]) + 1
    else:
        return count_vowels(word[1:])