Concept
In computer science, Backus–Naur form (ˌbækəs_ˈnaʊər) or Backus normal form (BNF) is a metasyntax notation for context-free grammars, often used to describe the syntax of languages used in computing, such as computer programming languages, document formats, instruction sets and communication protocols. It is applied wherever exact descriptions of languages are needed: for instance, in official language specifications, in manuals, and in textbooks on programming language theory.
Many extensions and variants of the original Backus–Naur notation are used; some are exactly defined, including extended Backus–Naur form (EBNF) and augmented Backus–Naur form (ABNF).
A BNF specification is a set of derivation rules, written as
::= expression
where:
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 a 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 .
As an example, consider this possible BNF for a U.S. postal address:
::=
::= |
::= "." |
::=
::= ","
::= "Sr." | "Jr." | | ""
::= | ""
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 suffix (Jr.