分词,保留网址url。
nltk.casual_tokenize('Enigma (now part of PTC http://www.linkedin.com/company/ptc)')
[u'Enigma', u'(', u'now', u'part', u'of', u'PTC', u'http://www.linkedin.com/company/ptc', u')']nltk.casual_tokenize('ROSS Corporate/Pivotal Retail Services')[u'ROSS', u'Corporate', u'/', u'Pivotal', u'Retail', u'Services']