Natural Language Processing with Python

Summary

• Sentences have internal organization that can be represented using a tree. Notable features of constituent structure are: recursion, heads, complements, and modifiers.

• A grammar is a compact characterization of a potentially infinite set of sentences; we say that a tree is well-formed according to a grammar, or that a grammar licenses a tree.

• A grammar is a formal model for describing whether a given phrase can be assigned a particular constituent or dependency structure.

• Given a set of syntactic categories, a context-free grammar uses a set of productions to say how a phrase of some category A can be analyzed into a sequence of smaller parts α₁ ... α_n.

• A dependency grammar uses productions to specify what the dependents are of a given lexical head.

• Syntactic ambiguity arises when one sentence has more than one syntactic analysis (e.g., prepositional phrase attachment ambiguity).

• A parser is a procedure for finding one or more trees corresponding to a grammatically well-formed sentence.

• A simple top-down parser is the recursive descent parser, which recursively expands the start symbol (usually S) with the help of the grammar productions, and tries to match the input sentence. This parser cannot handle left-recursive productions (e.g., productions such as NP -> NP PP). It is inefficient in the way it blindly expands categories without checking whether they are compatible with the input string, and in repeatedly expanding the same non-terminals and discarding the results.

• A simple ...