meta languages

Meta Languages in SDD

Software Design and Development

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2. EBNF2. EBNF

1. BNF1. BNF

3. Railroad diagram3. Railroad diagram

Course Specifications

NSW BOS Course Specifications

4. examples4. examples

BNF

Abbreviation for Backus-Naur form.

BNF is a metalanguage used to define the syntax of a programming language.

It uses the following symbols:

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BNFexamples

This translates into English as:

•A postal address consists of a name-part, followed by a street-address part, followed by a zip-code part. •A name-part consists of either: a personal-part followed by a last name followed by an optional "jr-part" (Jr., Sr., or dynastic number) and end-of-line, or a personal part followed by a name part (this rule illustrates the use of recursion in BNFs, covering the case of people who use multiple first and middle names and/or initials). •A personal-part consists of either a first name or an initial followed by a dot. •A street address consists of a house number, followed by a street name, followed by an optional apartment specifier, followed by an end-of-line. •A zip-part consists of a town-name, followed by a comma, followed by a state code, followed by a ZIP-code followed by an end-of-line.

Note that many things (such as the format of a first-name, apartment specifier, ZIP-code, and Roman numeral) are left unspecified here. If necessary, they may be described using additional BNF rules.

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EBNF

Abbreviation for extended Backus-Naur form.EBNF is a metalanguage used to define the syntax of a programming language.

In this extended form the following symbols are used:

Interpretation:An identifier is defined to be a Letter followed by one or more Letters or Digits.Letter is a non-terminal symbol and is defined elsewhere, and Digit is another non-terminal symbol defined elsewhere.

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EBNFexampleContents

Railroad diagram

This is an alternative, graphical method used to define the syntax of a programming language.

• Rectangles are used to enclose non-terminal symbols (that is, symbols that will be further defined).

• Circles or rounded rectangles are used to enclose terminal symbols.

These elements are linked by paths to show all legal combinations.

1. By starting at the left-hand side of the diagram and 2. tracing any path in a forward direction 3. to reach the right-hand side of the diagram, 4. a syntactically correct construct will be covered.

(‘Railroad’ in this context means a branch in the diagram is legal if it is treated as a set of points in a railroad layout and a train can take the branch in a forward direction only).

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Railroad diagramexampleContents

BNF, EBNF, Railroad Diagramexample

One possible set of syntax diagrams for this grammar is:

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