In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML, based on Murata Makoto's RELAX and James Clark's TREX.[1][2][3] 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; however, RELAX NG also offers a popular compact, non-XML syntax.[4] Compared to other popular schema languages, RELAX NG is relatively simple. It was defined by a committee specification of the OASIS RELAX NG technical committee in 2001 and 2002, and also by part two of the international standard ISO/IEC 19757: Document Schema Definition Languages (DSDL).[5][6] ISO/IEC 19757-2 was developed by ISO/IEC JTC1/SC34 (ISO/IEC Joint Technical Committee 1, Subcommittee 34 - Document description and processing languages) and published in its first version in 2003.[7]
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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:
<book> <page>This is page one.</page> <page>This is page two.</page> </book>
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:
<element name="book" xmlns="http://relaxng.org/ns/structure/1.0"> <oneOrMore> <element name="page"> <text/> </element> </oneOrMore> </element>
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:
<grammar xmlns="http://relaxng.org/ns/structure/1.0"> <start> <element name="book"> <oneOrMore> <ref name="page"/> </oneOrMore> </element> </start> <define name="page"> <element name="page"> <text/> </element> </define> </grammar>
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:
element book
{
element page { text }+
}
With named patterns, this can be flattened to:
start = element book { page+ }
page = element page { text }
A compact RELAX NG parser will treat these two as the same pattern.
Although the RELAX NG specification was developed at roughly the same time as the W3C XML Schema specification, the latter was arguably better known and more widely implemented in both open-source and commercial XML parsers and editors when it became a W3C Recommendation in 2001. Since then, however, RELAX NG support has increasingly found its way into XML software, and its acceptance has been aided by its adoption as a primary schema for popular document-centric markup languages such as DocBook, the TEI Guidelines, and OpenDocument.
RELAX NG shares with W3C XML Schema many features that set both apart from traditional DTDs: data typing, regular expression support, namespace support, ability to reference complex definitions.
By informal convention, RELAX NG schemas in the regular syntax are typically named with the filename extension ".rng". For schemas in the compact syntax, the extension ".rnc" is used.