THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

Example DTD

A DTD for our recipe collections, recipes.dtd:

<!ELEMENT collection (description,recipe*)> <!ELEMENT description ANY> <!ELEMENT recipe (title,ingredient*,preparation,comment?,nutrition)> <!ELEMENT title (#PCDATA)> <!ELEMENT ingredient (ingredient*,preparation)?> <!ATTLIST ingredient name CDATA #REQUIRED amount CDATA #IMPLIED unit CDATA #IMPLIED> <!ELEMENT preparation (step*)> <!ELEMENT step (#PCDATA)> <!ELEMENT comment (#PCDATA)> <!ELEMENT nutrition EMPTY> <!ATTLIST nutrition protein CDATA #REQUIRED carbohydrates CDATA #REQUIRED fat CDATA #REQUIRED calories CDATA #REQUIRED alcohol CDATA #IMPLIED>

By inserting:

<!DOCTYPE collection SYSTEM "recipes.dtd">

<!DOCTYPE collection SYSTEM "recipes.dtd">

in the headers of recipe collection documents, we state that they are intended to conform to recipes.dtd.

Alternatively, the DTD can be given locally with <!DOCTYPE collection [ ... ]>.

This grammatical description has some obvious shortcomings:

we cannot express that, e.g. protein, must contain a non-negative number
unit should only be allowed when amount is present
the comment element should be allowed to appear anywhere
nested ingredient elements should only be allowed when amount is absent