Compared to the alternatives, RELAX NG schemas are easy to use and learn, and the more you use them the more you become convinced.
RELAX NG (http://www.relaxng.org) is a powerful schema language with a simple syntax. Originally, RELAX NG was developed in a small OASIS technical committee led by James Clark. It is based on ideas from Clark’s TREX (http://www.thaiopensource.com/trex/) and Murata Makoto’s Relax (http://www.xml.gr.jp/relax/), and its first committee spec was published on December 3, 2001 (http://www.oasis-open.org/committees/relax-ng/spec.html). A tutorial is also available (http://www.oasis-open.org/committees/relax-ng/tutorial.html). Recently, RELAX NG became an international standard under ISO as ISO/IEC 19757-2:2004, Information technology— Document Schema Definition Language (DSDL)—Part 2: Regular-grammar-based validation—RELAX NG (see http://www.y12.doe.gov/sgml/sc34/document/0458.htm).
RELAX NG schemas may be written in either XML or a compact syntax. This hack demonstrates both.
Recall the document time.xml:
<?xml version="1.0" encoding="UTF-8"?> <!-- a time instant --> <time timezone="PST"> <hour>11</hour> <minute>59</minute> <second>59</second> <meridiem>p.m.</meridiem> <atomic signal="true"/> </time>
Here is a RELAX NG schema for time.xml called time.rng:
<element name="time" xmlns="http://relaxng.org/ns/structure/1.0"> <attribute name="timezone"/> <element name="hour"><text/></element> <element name="minute"><text/></element> ...