AB Check

def ABCheck(str):

    str = str.lower()
    next_a = -1
    next_b = -1

    for position, letter in enumerate(str):
        if position == next_a and letter == 'a' or position == next_b and letter == 'b':
            return True
        if letter == 'a':
            next_a = -1
            next_b = position + 4
        if letter == 'b':
            next_a = position + 4
            next_b = -1
        
    return False