6. 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 defined by a regular expression, or with an initial lower case letter otherwise. 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 in ISO/IEC 10646 whose canonical (UCS-4) code value, when interpreted as an unsigned binary number, has the value indicated. The number of leading zeros in the #xN form is insignificant; the number of leading zeros in the corresponding code value is governed by the character encoding in use and is not significant for XML.

					[a-zA-Z], [#xN-#xN]
				

matches any character with a value in the range(s) indicated (inclusive).

					[^a-z], [^#xN-#xN]
				

matches any character with a value outside the range indicated.

					[^abc], [^#xN#xN#xN]
				

matches any character with a value not among the characters given.

					"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.

					A | B
				

matches A or B but not both.

					A - B
				

matches any string that matches A but does not match B.

					A+
				

matches one or more occurrences of A.

					A*
				

matches zero or more occurrences of A.

Other notations used in the productions are:

					/* … */
				

comment.

					[ wfc: … ]
				

well-formedness constraint; this identifies by name a constraint on well-formed documents associated with a production.

					[ vc: … ]
				

validity constraint; this identifies by name a constraint on valid documents associated with a production.