Extended Backus-Naur Form (EBNF) as used by the W3C XML specification.

notation.md

Notation

The formal grammar of XML is given in this specification using a simple Extended Backus-Naur Form (EBNF) 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.