Backus-Naur-Form

The Backus-Naur form (BNF) – a meta-syntax notation for context-free grammars – is used to define grammars. For more information see Wikipedia.

A BNF specification is a set of derivation rules, written as

<symbol> ::= __expression__

where:

  • <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 a choice, the whole being a possible substitution for the symbol on the left,

  • symbols that never appear on a left side are terminals, while symbols that appear on a left side are non-terminals and are always enclosed between the pair of angle brackets <>,

  • terminals are enclosed with quotation marks: "text". "" is an empty string,

  • optional items are enclosed in square brackets: [<item-x>], or suffixed with an additional (questionmark) symbol, ?, such as <optional-symbol>?,

  • items existing 0 or more times are enclosed in curly brackets are suffixed with an asterisk (*) such as <word>::= <letter> {<letter>}*,

  • items existing 1 or more times are suffixed with an addition (plus) symbol, +, such as <word>::= {<letter>}+,

  • round brackets are used to explicitly define the order of expansion to indicate precedence, example: ( <symbol1> | <symbol2> ) <symbol3>,

  • text without quotation marks is an informal explanation of what is expected; this text is cursive if grammar is non-recursive and vice versa.

Example:

<contact-address> ::= <name> "e-mail addresses:" <e-mail-Addresses>

<e-mail-Addresses> ::= {<e-mail-Address>}*

<e-mail-Addresse> ::= <local-part> "@" <domain>

<name> ::= characters

<local-part> ::= characters conformant to local-part in RFC 5322

<domain> ::= characters conformant to domain in RFC 5322

Valid contact addresses:

Hugo Me e-mail addresses: Hugo@example.com

Hugo e-mail addresses: Hugo.Me@text.de

Invalid contact addresses:

Hugo

Hugo Hugo@ example.com

Hugo@example.com