BNF

What is BNF?

BNF is an acronym for "Backus Naur Form". John Backus and Peter Naur introduced for the first time a formal notation to describe the syntax of a given language (ALGOL 60 programming language).

See: RFC 2234.

BNF syntax

The meta-symbols of BNF are:

::=

Meaning "is defined as".

|

Meaning "or".

<, >

Angle brackets used to surround category names (some times skipped).

[, ]

Optional items are enclosed in.

{, }

Repetitive items (zero or more times) are enclosed in.

"abc..."

Terminals are enclosed in to distinguish them from meta-symbols.

BNF in BNF

syntax     ::=  { rule }
rule       ::=  identifier  "::="  expression
expression ::=  term { "|" term }
term       ::=  factor { factor }
factor     ::=  identifier |
                quoted_symbol |
                "("  expression  ")" |
                "["  expression  "]" |
                "{"  expression  "}"
identifier ::=  letter { letter | digit }
quoted_symbol ::= """ { any_character } """