def sentence_tokenize(sentence, spec_chars='.,;!?'):
    """ prepossess sentence, all as lower characters and remove special chars

    :param str sentence:
    :return [str]:

    >>> s = 'Hi there, how are you?'
    >>> sentence_tokenize(s)
    ['hi', 'there', 'how', 'are', 'you']
    """
    for char in spec_chars:
        sentence = sentence.replace(char, ' ')
    tokens = sentence.lower().split()
    return tokens