Backus-Naur Form

Last Updated: 07 Dec 2022

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The Backus-Naur Form, which is also called the BNF, the Backus Normal Form and or the Backus-Naur formalism, is a notation used in the description of a part of the syntax of “sentences” of a language. In about 1959, the Backus-Naur Form was suggested by John Backus, a constituent or part of the thirteen members which comprise the Algol 60 committee. Moreover, John Backus, besides from being a constituent of IBM, is also a major figure responsible for FORTRAN. The Backus-Naur Form has been used since then to describe the syntax of Algol 60. The BNF along wit its extensions has become standard tools for describing the syntax of programming notations, and in many cases, parts of compilers are generated automatically from a BNF description. The specification of the Backus-Naur Form includes a set of derivation rules. This set of deviation rules are written as “<symbol> :: = <expression with symbols>. The <symbol> in this formula is considered to be nonterminal.

The sequences of symbols create the entire expression. Furthermore, sequences separated by a vertical bar “|”, may also consist of the expression. The “|” sign is made use of in order to point out a definite choice. An expression which are not found on the left side, are considered to be terminals. In the present, Backus-Naur Form specifications are created in a form which it is easily read by humans and oftentimes, they are informal, including certain syntax rules and extensions. Syntax rules and extensions state that optional items should be enclosed in square brackets, like in “[<item-x>]” and that items which repeat or occur 0 or more times should be enclosed in curly brackets, such as in “<word> ::= <letter> { <letter> }. ” While, items which repeat or occur 1 or more times should be followed using a “+. ” Syntax rules and extension also state that the non-terminals should be written or typed using plain text, instead of using italics and angle brackets. While the terminals appear in bold. Optional choices in production should be separated through the use of the vertical bar symbol, like in “<alternative-A>| <alternative-B>. ” If an item in the Backus-Naur Form is repeated, an asterisk “*” should be placed after the item and that simple parenthesis is used, by enclosing the item, to group the items.