Валидация имени пользователя

login_validation.py

from re import match, I

def login_is_valid_re(login: str):
    return bool(
        match('^[a-z][a-z0-9\-\.]{0,19}', login, I) and \
        match('[a-z0-9\-\.]{0,19}[a-z0-9]$', login, I)
    )

def login_is_valid(login: str):
    head = 'abcdefghijklmnopqrstuvwxyz'
    tail = head + '0123456789'
    body = tail + '.-'
    _login = login.lower()

    return bool(
        1 <= len(login) <= 20 and
        _login[0] in head and
        _login[-1] in tail and
        [i for i in _login[1:-1] if i in body]
    )