1 .. -*- coding: utf-8; -*-

     2 .. include:: HEADER.rst

     3 

     4 =====

     5  BNF

     6 =====

     7 

     8 What is BNF?

     9 ============

    10 .. contents::

    11    :local:

    12 

    13 BNF is an acronym for "Backus Naur Form". John Backus and Peter Naur introduced

    14 for the first time a formal notation to describe the syntax of a given language

    15 (ALGOL 60 programming language).

    16 

    17 See: :rfc:`2234`.

    18 

    19 BNF syntax

    20 ==========

    21 

    22 The meta-symbols of BNF are:

    23 

    24   ``::=``

    25     Meaning "is defined as".

    26   ``|``

    27     Meaning "or".

    28   ``<``,  ``>``

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

    30   ``[``,  ``]``

    31     Optional items are enclosed in.

    32   ``{``,  ``}``

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

    34   ``"abc..."``

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

    36 

    37 BNF in BNF

    38 ==========

    39 ::

    40 

    41   syntax     ::=  { rule }

    42   rule       ::=  identifier  "::="  expression

    43   expression ::=  term { "|" term }

    44   term       ::=  factor { factor }

    45   factor     ::=  identifier |

    46                   quoted_symbol |

    47                   "("  expression  ")" |

    48                   "["  expression  "]" |

    49                   "{"  expression  "}"

    50   identifier ::=  letter { letter | digit }

    51   quoted_symbol ::= """ { any_character } """

    52