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La forma Backus-Naur (también conocida como *BNF, el formalismo de Backus-Naur, forma normal de Backus, o forma de Panini-Backus) es una mentasintaxis usada para expresar gramáticas independientes del contexto. Es una manera formal de desciribr lenguajes formales.

BNF se usa ampliamente como la notación para las gramáticas de los lenguajes de programación, juegos de instrucciones, y protocolos de comunicaciones, así como una notación para representar partes de las gramáticas de lenguajes naturales.

Existen diversas variaciones y ampliaciones sobre BNF. En este curso usaremos la definición de EBNF (Extended Backus-Naur Form) usada por la W3C (Wordld Wide Web Consortium) para la definición de XML (http://www.w3.org/TR/REC-xml/#sec-notation).

Each rule in the grammar defines one symbol, in the form

symbol ::= expression

Symbols are written with an initial capital letter if they are the start symbol of a regular language, otherwise with an initial lowercase letter. Literal strings are quoted.

Within the expression on the right-hand side of a rule, the following expressions are used to match strings of one or more characters:

• #xN
  where N is a hexadecimal integer, the expression matches the character whose number (code point) in ISO/IEC 10646 is N. The number of leading zeros in the #xN form is insignificant.

• [a-zA-Z], [#xN-#xN]
  matches any Char with a value in the range(s) indicated (inclusive).

• [abc], [#xN#xN#xN]
  matches any Char with a value among the characters enumerated. Enumerations and ranges can be mixed in one set of brackets.

• [^{a-z], [}#xN-#xN]
  matches any Char with a value outside the range indicated.

• [^{abc], [}#xN#xN#xN]
  matches any Char with a value not among the characters given. Enumerations and ranges of forbidden values can be mixed in one set of brackets.

• "string"
  matches a literal string matching that given inside the double quotes.

• 'string'
  matches a literal string matching that given inside the single quotes.

These symbols may be combined to match more complex patterns as follows, where A and B represent simple expressions:

• (expression)
  expression is treated as a unit and may be combined as described in this list.

• A?
  matches A or nothing; optional A.

• A B
  matches A followed by B. This operator has higher precedence than alternation; thus A B | C D is identical to (A B) | (C D).

• A | B
  matches A or B.
• A - B
  matches any string that matches A but does not match B.

• A+
  matches one or more occurrences of A. Concatenation has higher precedence than alternation; thus A+ | B+ is identical to (A+) | (B+).

• A*
  matches zero or more occurrences of A. Concatenation has higher precedence than alternation; thus A* | B* is identical to (A*) | (B*).

Other notations used in the productions are:

• /* ... */
  comment.

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