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Natural Language Processing with Python by Edward Loper, Steven Bird, Ewan Klein

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Chapter 8. Analyzing Sentence Structure

Earlier chapters focused on words: how to identify them, analyze their structure, assign them to lexical categories, and access their meanings. We have also seen how to identify patterns in word sequences or n-grams. However, these methods only scratch the surface of the complex constraints that govern sentences. We need a way to deal with the ambiguity that natural language is famous for. We also need to be able to cope with the fact that there are an unlimited number of possible sentences, and we can only write finite programs to analyze their structures and discover their meanings.

The goal of this chapter is to answer the following questions:

  1. How can we use a formal grammar to describe the structure of an unlimited set of sentences?

  2. How do we represent the structure of sentences using syntax trees?

  3. How do parsers analyze a sentence and automatically build a syntax tree?

Along the way, we will cover the fundamentals of English syntax, and see that there are systematic aspects of meaning that are much easier to capture once we have identified the structure of sentences.

Some Grammatical Dilemmas

Linguistic Data and Unlimited Possibilities

Previous chapters have shown you how to process and analyze text corpora, and we have stressed the challenges for NLP in dealing with the vast amount of electronic language data that is growing daily. Let’s consider this data more closely, and make the thought experiment that we have a gigantic corpus consisting ...

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