text = """
Punkt knows that the periods in Mr. Smith and Johann S. Bach
do not mark sentence boundaries.  And sometimes sentences
can start with non-capitalized words.  i is a good variable
name.
"""
sent_detector = nltk.data.load('tokenizers/punkt/english.pickle')
print('\n-----\n'.join(sent_detector.tokenize(text.strip())))
Несколько отрицательных утверждений просмотра назад в регулярном выражении python?