title: How to parse sentences based on lexical content (phrases) with Python-NLTK
body: <p>Can Python-NLTK recognize input string and parse it not only based on white space but also on the content? Say, "computer system" became a phrases in this situation. Can anyone provide a sample code?</p>
  
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  <p><strong>input String</strong>: "A survey of user opinion of computer system response time"</p>
  
  <p><strong>Expected output</strong>: ["A", "survey", "of", "user", "opinion", "of", "computer system", "response", "time"]</p>