2

I read Extensible Markup Language (XML) 1.0 (Fifth Edition) W3C Recommendation 26 November 2008
3.2 Element Type Declarations has: An element type declaration takes the form: Element Type Declaration

elementdecl ::= <!ELEMENT Name contentspec >
contentspec ::= 'EMPTY' | 'ANY' | Mixed | children

And in 3.2.1 Element Content has: Element-content Models

children ::= (choice | seq) ('?' | '*' | '+')?
cp ::= (Name | choice | seq) ('?' | '*' | '+')?
choice ::= '(' S? cp ( S? '|' S? cp )+ S? ')'
 seq ::=    '(' S? cp ( S? ',' S? cp )* S? ')'

After it I had question. Which different between 'contentspec' and 'content model'.

Maybe contentspec is ANY, PCDATA, Mixed, children.

And only children has 'content model': (elemName1 | elemName2, elemName3, elemET).

(Name | , '?' '*' '+'), sequence, choice - all that is 'content model'. Right?

Does Mixed have 'content model'?

In tutorial write often:

<!ELEMENT Name content_model >
user3771600
  • 33
  • 1
  • 6

1 Answers1

0

You got it.

contentspec is ANY, PCDATA, Mixed, children. And only children has 'content model'

It describes all the types of content an element can have:

  • EMPTY tag -- no content
  • ANY any other elements defined in the DTD -- kind of free-form
  • Mixed content, which is decribed in 3.2.2 of the XML recommendation
  • children, which is described in 3.1.1 of the XML recommendation

Content model allows to use sequence, choice, parenthis and so on, e.g. ((a|b)|(c+, d?, e*))?, and only reference others elements -- #PCDATA is not allowed here.

Mixed content is a bit particular since this model can only use choice, and therefore is distinct to content model. A mixed content is either #PCDATA or something like (#PCDATA | a | b | c)*. In the latter case you need to open an parenthesis (, #PCDATA must come first, then you specify the allowed elements separated by the choice | and finish by closing the parenthesis ), with the zero or more occurence specifier *.

The consequence of all this is provided in 3.2.2 Mixed content :

In this case, the types of the child elements may be constrained, but not their order or their number of occurrences

In particular, it won't be possible to define an element:

  • that can contain either text (#PCDATA) or a sequence of elements: e.g. (#PCDATA) | (a, b, c)) is not valid
  • that must start with an element, followed by text, then other element: e.g. (a, #PCDATA;, b, c) is also not valid

You can not also be ensured that your element in your XML instance will have any content at all (it can remain empty).

potame
  • 7,597
  • 4
  • 26
  • 33