DTD. Element Type Declaration. Different between 'contentspec' and 'content model'

问题

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 >

回答1:

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).

来源：https://stackoverflow.com/questions/24389351/dtd-element-type-declaration-different-between-contentspec-and-content-mode

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