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4.1
Syntactic Analysis

The second part of a natural language understanding system consists of syntactic analysis. Sequences of words are transformed into structures that show how the words relate to each other [16]. Some of the phrases may be rejected if they form a phrase that is not allowed in the grammer. In order to begin to extract meaning from the sentence, it must be broken down into organizational patterns that a machine can understand. This process is called parsing.

Parsing techniques are based on the theory of formal languages. These are mathematical abstractions that can be used in modeling the syntax of natural languages. "A formal language is defined in terms of an alphabet and a grammar that determines the ways in which symbols of the alphabet may be combined into sentences," explains Steven Tanimoto [17].

A particular grammar is specified by describing the following components:

  1. An alphabet of terminal symbols. These appear in the sentences to be parsed.
  2. An alphabet of nonterminal symbols, that are used during the process of generating intermediate strings, but they do not appear in the input sentences.
  3. A start symbol that belongs to one of the nonterminal symbols.
  4. A finite set of production rules, each of which consists of a left-hand side string and a right-hand side string.
To test if a sentence was produced by the grammar, the system starts with the start symbol and applies the production rules until the input sentence is produced.