with open("domains.txt", "r") as dirty_domains:
    lines = dirty_domains.readlines()
    clean_domains = []

    for domain in lines:
        domain = domain.replace("\n", "")
        without_sub = re.search(r'(?<=\.).*\..*', domain)

        if(without_sub != None):
            clean_domains.append(without_sub.group(0))
        else:
            clean_domains.append(domain)

with open("clean_domains.txt", "w") as filehandle:
    for domain in clean_domains:
        filehandle.write('%s\n' % domain)