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−.. -*- coding: utf-8; -*-
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−
+
−=====
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− BNF
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−=====
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−
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−What is BNF?
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−============
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−.. contents::
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−   :local:
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−
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−BNF is an acronym for "Backus Naur Form". John Backus and Peter Naur introduced
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−for the first time a formal notation to describe the syntax of a given language
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−(ALGOL 60 programming language).
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−
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−See: :rfc:`2234`.
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−
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−BNF syntax
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−==========
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−
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−The meta-symbols of BNF are:
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−
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−``::=``
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−  Meaning "is defined as".
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−``|``
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−  Meaning "or".
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−``<``,  ``>``
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−  Angle brackets used to surround category names (some times skipped).
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−``[``,  ``]``
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−  Optional items are enclosed in.
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−``{``,  ``}``
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−  Repetitive items (zero or more times) are enclosed in.
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−``"abc..."``
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−  Terminals are enclosed in to distinguish them from meta-symbols.
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−
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−BNF in BNF
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−==========
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−::
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−
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−  syntax     ::=  { rule }
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−  rule       ::=  identifier  "::="  expression
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−  expression ::=  term { "|" term }
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−  term       ::=  factor { factor }
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−  factor     ::=  identifier |
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−                  quoted_symbol |
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−                  "("  expression  ")" |
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−                  "["  expression  "]" |
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−                  "{"  expression  "}"
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−  identifier ::=  letter { letter | digit }
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−  quoted_symbol ::= """ { any_character } """
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−