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.. include:: HEADER.rst

=====
 BNF
=====

What is BNF?
============
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   :local:

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 } """