import re


def clean(sentence):
    """
    Clean up a sentence. Remove non alpha number letters. Lower cased.

    >>> clean('all done.')
    'all done'
    >>> clean('ALL DONE.')
    'all done'
    >>> clean('here we go:')
    'here we go'

    :param sentence:
    :return:
    """
    return re.sub('[^A-Za-z0-9]', ' ', sentence).lower().strip()