RELAX NG
| Filename extension | .rng |
|---|---|
| Internet media type | application/xml, text/xml |
| Type of format | XML Schema language |
| Extended from | XML |
In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML - a RELAX NG schema specifies a pattern for the structure and content of an XML document. A RELAX NG schema is itself an XML document but RELAX NG also offers a popular compact, non-XML syntax. Compared to other XML schema languages RELAX NG is considered relatively simple.
It was defined by a committee specification of the OASIS RELAX NG technical committee in 2001 and 2002, based on Murata Makoto's RELAX and James Clark's TREX, and also by part two of the international standard ISO/IEC 19757: Document Schema Definition Languages (DSDL). ISO/IEC 19757-2 was developed by ISO/IEC JTC1/SC34 and published in its first version in 2003.
Suppose we want to define an extremely simple XML markup scheme for a book: a book is defined as a sequence of one or more pages; each page contains text only. A sample XML document instance might be:
A RELAX NG schema can be written in a nested structure by defining a root element that contains further element definitions, which may themselves contain embedded definitions. A schema for our book in this style, using the full XML syntax, would be written:
Nested structure becomes unwieldy with many sublevels and cannot define recursive elements, so most complex RELAX NG schemas use references to named pattern definitions located separately in the schema. Here, a "flattened schema" defines precisely the same book markup as the previous example:
RELAX NG compact syntax is a non-XML format inspired by extended Backus-Naur form and regular expressions, designed so that it can be unambiguously translated to its XML counterpart, and back again, with one-to-one correspondence in structure and meaning, in much the same way that Simple Outline XML (SOX) relates to XML. It shares many features with the syntax of DTDs. Here is the compact form of the above schema:
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