RELAX NG enables you to create a customized subset or extension of a DTD much more easily than doing it the old-fashioned way.
You may find at some point in your XML work—especially if you’re working with a large, complex XML vocabulary such as DocBook (http://www.docbook.org)—that you don’t use or want the full set of elements that the XML vocabulary provides, and would much rather work with a smaller, custom subset that excludes the elements you don’t need.
If you’re working with a RELAX NG schema,
you’ll be happy to find out that making a subset of
it is a relatively trivial task, especially if
you’re using a RELAX NG compact syntax (RNC) schema
instead of a RELAX NG XML-syntax (RNG) schema. You basically just
need to create a file in which you include (by reference) the schema
you want to subset, and then list out all the elements you want to
exclude, giving the
notAllowed pattern for the
content of the element.
On the other hand, if you’re working with a DTD, you’ll find that trying to create a subset of it the old-fashioned way (by making a DTD customization layer or by editing the DTD directly) can be a much more complicated and time-consuming task.
The good news is that you don’t need to do it the old-fashioned way because you can use some existing free software to quickly and easily generate a custom DTD: Norm Walsh’s Perl script flatten.pl (http://cvs.sourceforge.net/viewcvs.py/*checkout*/docbook/cvstools/flatten), James Clark’s ...