def last_word(text, include='alphanum_underscore'):
    """
    Find the last word in a sentence.
    >>> last_word('abc')
    'abc'
    >>> last_word(' abc')
    'abc'
    >>> last_word('')
    ''
    >>> last_word(' ')
    ''
    >>> last_word('abc ')
    ''
    >>> last_word('abc def')
    'def'
    >>> last_word('abc def ')
    ''
    >>> last_word('abc def;')
    ''
    >>> last_word('bac $def')
    'def'
    >>> last_word('bac $def', include='most_punctuations')
    '$def'
    >>> last_word('bac \def', include='most_punctuations')
    '\\\\def'
    >>> last_word('bac \def;', include='most_punctuations')
    '\\\\def;'
    >>> last_word('bac::def', include='most_punctuations')
    'def'
    """

    if not text:   # Empty string
        return ''

    if text[-1].isspace():
        return ''
    else:
        regex = cleanup_regex[include]
        matches = regex.search(text)
        if matches:
            return matches.group(0)
        else:
            return ''
Find the last word in a sentence.
    >>> last_word('abc')
    'abc'
    >>> last_word(' abc')
    'abc'
    >>> last_word('')
    ''
    >>> last_word(' ')
    ''
    >>> last_word('abc ')
    ''
    >>> last_word('abc def')
    'def'
    >>> last_word('abc def ')
    ''
    >>> last_word('abc def;')
    ''
    >>> last_word('bac $def')
    'def'
    >>> last_word('bac $def', include='most_punctuations')
    '$def'
    >>> last_word('bac \def', include='most_punctuations')
    '\\\\def'
    >>> last_word('bac \def;', include='most_punctuations')
    '\\\\def;'
    >>> last_word('bac::def', include='most_punctuations')
    'def'