'Compiles and returns a regular expression to find contexts
including possible sentence boundaries.'

def period_context_re(self):

    try:
        return self._re_period_context
    except:
        self._re_period_context = re.compile((self._period_context_fmt % {u'NonWord': self._re_non_word_chars, u'SentEndChars': self._re_sent_end_chars}), (re.UNICODE | re.VERBOSE))
        return self._re_period_context
