XML - 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.



[ 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.