def alternate(s):
    valid_substrings = []
    for first, second in combinations(set(s), 2):
        pattern = "[^" + first + second + "]"
        substring = re.sub(pattern, '', s)
        if not re.search(r'([a-z])\1', substring):
            valid_substrings.append(substring)
    return max([len(i) for i in valid_substrings]) if valid_substrings else 0