6 Notation
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 match that
given inside the double quotes.
-
'string'
-
matches a literal string match 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.
-
[ 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 Validity
documents associated with a production.
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