title: Python: NLTK - regexp tokenizer produces empty output
body: <p>Im trying to tokenize the text available on NLTK text book (using python 2.7) but the output is not as expected. Is there something I am missing?</p>
  
  <pre><code>text = 'That U.S.A. poster-print costs $12.40...'
  
  pattern = r'''(?x)     # set flag to allow verbose regexps
     ([A-Z]\.)+          # abbreviations, e.g. U.S.A.
     | \w+(-\w+)*        # words with optional internal hyphens
     | \$?\d+(\.\d+)?%?  # currency and percentages, e.g. $12.40, 82%
     | \.\.\.            # ellipsis
     | [][.,;"'?():-_`]  # these are separate tokens; includes ], [
     '''
  
  nltk.regexp_tokenize(text, pattern)
  
  
  Output: 
   [('', '', ''),
   ('A.', '', ''),
   ('', '-print', ''),
   ('', '', ''),
   ('', '', '.40'),
   ('', '', '')]
  
  Expected:
  ['That', 'U.S.A.', 'poster-print', 'costs', '$12.40', '...']
  </code></pre>