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Re: DTD vs XSD: No Duplicate Types in (Mixed) Content Models

• From: "G. Ken Holman" <gkholman@CraneSoftwrights.com>
• To: xml-dev@lists.xml.org,xmlschema-dev-request@w...
• Date: Mon, 31 Jan 2011 08:36:09 -0500

At 2011-01-31 13:56 +0100, Maik Stührenberg wrote:
>One of my students asked me if it wouldn't be 
>easy to simulate XML Schema's minOccurs and 
>maxOccurs by repeating the same element type 
>(element name) in the content model of its parent element, e.g.:
>
><!ELEMENT a (b, b, b)>
><!ELEMENT b EMPTY>

...

>All three solutions were tolerated by the 
>parsers I've tried. For simulating 
>minOccurs="3", maxOccurs="5" one could use the following DTD solution:
>
><!ELEMENT a (b, b, b, b?, b?)>
><!ELEMENT b EMPTY>
>
>To my suprise, I had no problems with the parsers I've tried.

That surprises me as well, for the very reason you cited:

>The interesting case is, if I have exact four 
>occurrences of the element b in an instance, 
>resulting that my parser cannot know which b in the model is being matched.

Exactly.

>Was this behaviour expected?

Yes, I believe so, because of the ambiguous particles ... which way to go.

But have you tried, for minOccurs="3" and 
maxOccurs="5" the W3C Schema equivalent of the following:

    <!ELEMENT a (b, b, b, (b, b?)?)>

That combination is not ambiguous but gives you 
what you want.  There is no doubt which of the 
<b> elements is being encountered.

I hope this helps.

. . . . . . . . Ken

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