EBNF

EBNF is an extension of the Backus Naur Form (BNF) of syntax description, and was introduced by Niklaus Wirth in his article: What Can We Do About the Unnecessary Diversity of Notation for Syntatic Definitions which appeared in the November 1977 edition of the Communications of the ACM (volume 20, number 11, pages 822-823).

EBNF allows the many productions corresponding to a non-terminals in typical BNF grammars to be collapsed into one production by adding the following meta-symbols:

• () brackets to indicate once. (Used to group alternatives together.)
• [] brackets to indicate zero or once.
• {} brackets to indicate zero or more.
• | to separate alternatives.

In keeping with tradition, and despite the title of Wirth's article, the formalism used in this hypertext differs somewhat from all other syntax formalisms. Principally:

• Terminal words are not given in quotes to distinguish them from non-terminal names, since non-terminals are usually linked to other texts and thus look different anyway.
• When a terminal symbol needs to be distinguised from a meta symbol, the terminal symbol is made into a link.

For example, in:

[| | where] condition

the first bar is a terminal symbol and the second a meta symbol, and the first word is a terminal while the second is a non-terminal.

See also: Using EBNF to generate compilers.