Backus-Naur form

Backus-Naur Form(Q211577), often abbreviated as BNF, is one of the two main notation techniques for context-free grammars in computer science.

BNF is a formal language that can be denoted in a collection of key-value pairs. For instance:

  <symbol> ::= __expression__

In the example shown above, <symbol> denotes a key, and the __expression__ denotes a value.

• <symbol>is a nonterminal (variable) and the __expression__ consists of one or more sequences of either terminal or nonterminal symbols;
• =::= means that the symbol on the left must be replaced with the expression on the right.
• more sequences of symbols are separated by the vertical bar "|", indicating an alternation choice, the whole being a possible substitution for the symbol on the left.

Symbols that never appear on a left side are terminals. On the other hand, symbols that appear on a left side are non-terminals and are always enclosed between the pair <>.