DTD Question - PCDATA followed by an optional element

[Rick Jelliffe]

From: Richard Tobin <richard at cogsci.ed.ac.uk>

>> I have a large number of fields that normally hold simple textual
data, but
>> might also include a comment.
>
>You can't constrain the content in that way with a DTD.  The best you
>do is
>
>    <!ELEMENT rho0 (#PCDATA|comment)*>
>
>which allows any mixture of comments and text.

To get strong typing you either must

1)explicitly label the text section:
    <!ELEMENT rho0 (text, comment?)>
    <!ELEMENT text (#PCDATA)>

or
2) use some other schema language to validate: use a DTD with Richard's
    <!ELEMENT rho0 (#PCDATA|comment)*>

Then the following Schematron schema to check that only one comment is
used (I think this is correct, I haven't run it):
<schema><title>Norman's Schema</title>
    <pattern name="Rho0 Pattern">
        <rule context="rho0">
            <assert text="count(comment) &lt;= 1"
            >An rh0 element should have only one comment</assert>
        </rule>
    </pattern>
</schema>

You can download schematron-report, which is an error browser generating
HTML pages, from
http://www.ascc.net/xml/resource/schematron/schematron.html

Rick Jelliffe


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