Backus–Naur form

Example[edit]

As an example, consider this possible BNF for a U.S. postal address:

 <postal-address> ::= <name-part> <street-address> <zip-part>      <name-part> ::= <personal-part> <last-name> <opt-suffix-part> <EOL>                     | <personal-part> <name-part>  <personal-part> ::= <initial> "." | <first-name> <street-address> ::= <house-num> <street-name> <opt-apt-num> <EOL>       <zip-part> ::= <town-name> "," <state-code> <ZIP-code> <EOL><opt-suffix-part> ::= "Sr." | "Jr." | <roman-numeral> | ""    <opt-apt-num> ::= <apt-num> | ""

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., Sr., or dynastic number) and end-of-line, or a personal part followed by a name part (this rule illustrates the use of recursion in BNFs, covering the case of people who use multiple first and middle names and initials).
• A personal-part consists of either a first name or an initial followed by a dot.
• A street address consists of a house number, followed by a street name, followed by an optional apartment specifier, followed by an end-of-line.
• A zip-part consists of a town-name, followed by a comma, followed by a state code, followed by a ZIP-code followed by an end-of-line.
• An opt-suffix-part consists of a suffix, such as "Sr.", "Jr." or a roman-numeral, or an empty string (i.e. nothing).
• An opt-apt-num consists of an apartment number or an empty string (i.e. nothing).

Note that many things (such as the format of a first-name, apartment specifier, ZIP-code, and Roman numeral) are left unspecified here. If necessary, they may be described using additional BNF rules.