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Non-Terminal Symbols (Syntax Parsing Engine)

Topic Overview

Purpose

This topic explains a grammar’s non-terminal symbols.

Required background

The following topics are prerequisites to understanding this topic:

Topic Purpose

This topic provides an overview of the Syntax Parsing Engine.

This topic provides an overview of the Syntax Parsing Engine’s Grammar.

This topic explains the lexical analysis performed by the Syntax Parsing Engine.

In this topic

This topic contains the following sections:

Non-Terminal Symbols Overview

Non-terminal symbols summary

Non-terminal symbols represent groupings of textual elements. Like terminal symbols, non-terminal symbols have a unique name so the parser can distinguish them from other symbols. This name must be unique across all terminal and non-terminal symbols in the same grammar.

In addition to this name, the non-terminal symbol also defines sequences of symbols it can represent in the document. Each non-terminal symbol can represent one or more sequences of symbols. For example, a variable declaration in C# can be created in various ways:

  • A type followed by an identifier followed by a semicolon.

  • A type followed by an identifier followed by an equals sign followed by an expression followed by a semicolon.

  • The “var” keyword followed by an identifier followed by an equals sign followed by an expression followed by a semicolon.

  • And so on…

The same non-terminal symbol can represent all these variations. If one of these sequences of symbols is found in a document in a place where a variable declaration is expected, the variable declaration non-terminal symbol can be used to represent those symbols.

Each sequence of symbols a non-terminal symbol can represent is called a production. A production is typically written with the non-terminal symbol, known as the head of the production, on the left of an arrow and the sequence of symbols, known as the body of the production, on the right. For example, these might be three productions for a variable declaration symbol:

VariableDeclarationStatement → Type IdentifierToken SemicolonToken

VariableDeclarationStatement → Type IdentifierToken EqualsToken Expression SemicolonToken

VariableDeclarationStatement → VarKeyword IdentifierToken EqualsToken Expression SemicolonToken

And there may be others as well. The productions tell the syntax analyzer the ways in which non-terminal symbols can represent groups of symbols so it can go about constructing a hierarchical syntax tree representing a document.

The productions for a non-terminal symbol, and therefore the sequences of symbols it can represent, are defined by a rule tree exposed by the Rule property on the NonTerminalSymbol class.

Related Content

Topics

The following topics provide additional information related to this topic.

Topic Purpose

This topic explains a Grammar’s terminal symbols.

This topic explains the syntax analysis performed by the Syntax Parsing Engine.

This topic explains the grammar analysis performed by the Syntax Parsing Engine.