(read as rewrite A as α), where α is a string of terminals and/or nonterminals. These productions apply irrespective of the context of A. For brevity one writes A → α1 | α2 | .. .. | α n
to indicate the separate productions A → α1, A → α2, .., .., A → αn
As an example, the following generates a simple class of arithmetic expressions typified by (a + b) × c:
E → T | T + E | (E)
T → E | E × T | a | b | c
The BNF notation used in defining the syntax of programming languages is simply a context-free grammar.
Context-free grammars are a class of phrase-structure grammar (PSG). GPSG represents the principal attempt at constructing context-free grammars capable of characterizing the grammars of natural language.
Compare context-sensitive grammar.
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