RELAX NG

Foreword by James Clark

It is a pleasure to see this first book on RELAX NG, particularly as—at the time of writing—RELAX NG does not enjoy the same level of usage and corporate support as its main rival, W3C XML Schema. Clearly this book is not for those who like their technologies chosen for them by major vendors. But I believe that for those who prefer to select their technologies for themselves, RELAX NG has substantial utility.

Back in February 1998, when the XML 1.0 Recommendation was first introduced, XML was radical in its simplicity compared to SGML. The innovation in XML was not so much in what it added to SGML but rather in what it took away. However, XML is now part of a much larger family of standards from the W3C. Collectively, these are much more complex than SGML ever was. It is hard for a newcomer to understand what is the right way to use XML and what are the core ideas.

RELAX NG is based on a very clear vision of XML processing. XML is useful only because XML processing components can interoperate. Most XML processing components do not input and output arbitrary XML documents. To combine XML processing components reliably, it is therefore essential to be able to specify the inputs and outputs of XML processing components and to verify mechanically that components are behaving according to their specifications. The most important issue in doing this is choosing which abstraction of XML to use for specifying the inputs and outputs of XML processing components.

XML ...