Relax NG, a Schema Language for XML

RELAX NGA Schema Language for XML

Michael Fitzgerald

[email protected]

Wy’east Communications

9 July 2003 Slide 2

Introductions

A brief, swift, technical overview of RELAX NG

Some comparisons with DTDs and W3C XML Schema

As John Cowan has said: “Once RELAX NG crosses the ‘blood-brain barrier,’ you can never go back!”

9 July 2003 Slide 3

What Is RELAX NG?

RELAX NG is a schema language for XML A schema language for XML describes constraints

for a vocabulary beyond ordinary XML syntax RELAX NG is simple, intuitive, elegant, easy to

use and learn, and has a foundation in finite tree automata

RELAX NG is being forwarded as part of ISO/IEC 19757 Document Schema Definition Languages or DSDL (see http://www.dsdl.org)

9 July 2003 Slide 4

When & Who?

Version 1.0 specs were developed by the RELAX NG technical committee at OASIS between April and December 2001

Merges Murata Makoto’s RELAX and James Clark’s TREX

James Clark is chair of the RELAX NG technical committee

9 July 2003 Slide 5

An Elegant Alternative

Murata Makoto coauthored the RELAX NG tutorial and specification with James Clark

Offers an XML as well as a compact, non-XML syntax

Committed to simplicity, modularity, composability

No side effects, no PSVI An alternative to the dominant W3C XML

Schema

9 July 2003 Slide 6

XML Schema Competition?

Is RELAX NG poised to threaten XML Schema’s dominance? No.

Will RELAX NG replace XML Schema? No.

Will RELAX NG continue to attract seasoned schema developers based on word of mouth? Probably.

9 July 2003 Slide 7

A Few Preliminaries

Patterns describe content and structure of instances

An instance of a schema is a document that complies with that schema

Most RELAX NG patterns can act as a document element for the schema

Structure namespace also denotes version: http://relaxng.org/ns/structure/1.0

9 July 2003 Slide 8

DTDs & RELAX NG

RELAX NG is an evolution of the DTD Elements and other structures are defined,

not declared No concept of associating a schema with an

instance, as with a document type declaration, as in:

<!DOCTYPE date SYSTEM "date.dtd">

9 July 2003 Slide 9

Element Definitions

The <element> element Must have a name attribute or a <name>

child element In compact syntax, defined with an element keyword

XML Schema likewise has an <xs:element> element

9 July 2003 Slide 10

For Example, Elements & Schemas

Following is an element definition which is also a complete schema in XML syntax (date.rng):

<element name="date" xmlns="http://relaxng.org/ns/structure/1.0">

<text/>

</element>

Compact syntax, namespace assumed by default (date.rnc):

element date { text }


Validating with Jing

Instance (date.xml):

<date>2003-07-09</date>

Jing is a multi-platform RELAX NG validator, written by James Clark, in Java

Validation the element examples:jing date.rng date.xml

jing –c date.rnc date.xml


Other RELAX NG Tools

James Clark’s Trang, a schema translator (http://thaiopensource.com/relaxng/trang.html)

Sun’s Multi-schema Validator or MSV (http://wwws.sun.com/software/xml/developers/

multischema/) Asami Tomoharu’s Relaxer schema

compiler (http://www.relaxer.org)


Adding an Attribute

Content models formed by simple nesting The <attribute> element is an example In XML syntax, <text/> is assumed as a

child of <attribute> and can be left out Compact syntax, however, requires the text keyword

XML Schema likewise uses an <xs:attribute> element


Attributes Examples XML syntax (att.rng):

<element name="date" xmlns="http://relaxng.org/ns/structure/1.0"> <attribute name="type"/> <text/></element>

Compact syntax (att.rnc):element date { attribute type { text }, text}

Match (att.xml):<date type="ISO">2003-07-09</date>


Empty Elements

Empty elements in XML may have attributes but no text or child element content

Element definitions may not be entirely empty If no attributes are defined, must use <empty/>

or empty in the content of the element definition XML Schema signals empty content by the

absence of a content model


Empty Elements Examples XML syntax (empty.rng):

<element name="br" xmlns="http://relaxng.org/ns/structure/1.0">

<empty/> </element> Compact (empty.rnc):

element br {empty} XML syntax (image.rng):

<element name="image" xmlns="http://relaxng.org/ns/structure/1.0">

<attribute name="source"/></element>

Compact (image.rnc): element image {attribute source {text}}

Match empty.xml, image.xml


Namespaces

In RNG, the ns attribute defines the namespace that the pattern should match

ns namespace matches a namespace defined in the instance with xmlns

ns is inherited by child elements xmlns declares, ns defines the matching

namespace Compact syntax uses default and namespace

keywords


Namespaces Examples XML syntax (ns.rng):

<element name="date" ns="http://www.wyeast.net/date" xmlns="http://relaxng.org/ns/structure/1.0"> <text/></element>

Compact (ns.rnc): default namespace = "http://www.wyeast.net/date"element date {text}

Match (ns.xml): <date xmlns="http://www.wyeast.net/date">2003-07-09</date>


Occurrence Constraints

<optional> is equivalent to ? (zero or one) in DTDs

<oneOrMore> is equivalent to + in DTDs <zeroOrMore> is equivalent to * in

DTDs ? and + and * work in RNC RELAX NG does not have minOccurs

and maxOccurs equivalents


One or More Examples XML syntax (dates.rng):

<element name="dates" xmlns=“http://relaxng.org/ns/structure/1.0"> <oneOrMore> <element name="date"><text/></element> </oneOrMore></element>

Compact (dates.rnc): element dates {element date {text}*}

Match (dates.xml): <dates> <date>2003-07-07</date> <date>2003-07-08</date> <date>2003-07-09</date></date>


choice & group

<choice> matches instances with any one of its children

Compact uses | as in DTDs <group> matches instances with all of its

children Compact uses () as in DTDs XML Schema also uses xs:choice and xs:group


choice & group Examples XML syntax (instant.rng):

<element name="instant"><choice><group><element name="date"><text/></element><element name="time"><text/></element>

</group><element name="date-time"><text/></element></choice>

</element> Compact (instant.rnc):

element instant {(element date {text}, element time {text})| element date-time {text}}


Definitions

Create named definitions with <define> Can refer to named definition with <ref> No name conflict with name of definition or

name of element or attribute Similar to <complexType> in XML

Schema


Definition Examples

XML syntax (see def.rng):<define name="date">

<element name="date"><element name="year"><text/></element>

<element name="month"><text/></element><element name="day"><text/></element></element>

</define>

Compact syntax (see def.rnc):date = element date {element year {text},

element month {text}, element day {text}}


Grammar

If <define> elements are used, both <grammar> and <start> must be used as well

<grammar> becomes root element for schema

<start> indicates document element in the instance (similar to what DOCTYPE does)


grammar, start & ref Example

XML syntax (def.rng/.rnc with def.xml):<grammar xmlns="http://relaxng.org/ns/structure/1.0"><start>

<ref name="date"/></start><define name="date">

<element name="date"><element name="year"><text/></element>

<element name="month"><text/></element><element name="day"><text/></element></element>

</define></grammar>


Datatypes

RELAX NG supports external datatype libraries, namely XML Schema datatypes

The datatypeLibrary attribute indicates the namespace for the datatype library (inherited)

The <data> element with the type attribute <param>, child element of <data>, indicates

facets of datatype per XML Schema


Datatypes in Compact Syntax

The datatype library is automatically declared for XML Schema datatypes

xsd prefix is required unless XML Schema datatypes is redeclared

parameters are defined with literal strings


Datatype Examples XML syntax (year.rng with year.xml):

<element name="year" xmlns="http://relaxng.org/ns/structure/1.0">datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes"> <data type="gYear">

<param name="minInclusive">2002</param> <param name="maxInclusive">2005</param> </data>

</element>

Compact syntax (year.rnc also with year.xml):element year {

xsd:gYear {minInclusive="2002" maxInclusive="2005"}}


Enumerations

In DTDs, enumerations are only possible in attributes, defined in a DTD like this:

<!ATTLIST week day (m|w|f) #REQUIRED>

Enumerations possible in RELAX NG in both elements and attributes

RELAX NG uses the <value> element in XML syntax, literals separated by | in compact syntax

XML Schema uses the <xs:enumeration> facet element, which is a child of <xs:restriction>, which is a child of <xs:simpleType>


Enumeration Examples

XML syntax (day.rng with day.xml):<element name="day"

xmlns="http://relaxng.org/ns/structure/1.0"><choice><value>m</value><value>w</value><value>f</value></choice>

</element>

Compact syntax (day.rnc also with day.xml):element day { "m" | "w" | "f" }


Lists

A list is whitespace-separated sequence of tokens

RELAX NG uses the <list> element to define a list, followed by one or more <data> elements

Compact syntax uses the list keyword followed by a comma separated list of types


More on Lists

Can use occurrence constraints such as <optional> or ?, <oneOrMore> or +, <zeroOrMore> or *

DTDs use NMTOKENS, IDREFS, and ENTITIES, but for attributes only

XML Schema uses <xs:list>, a child of <xs:simpleType>, which can be constrained by facets


List Examples

XML syntax (vertex.rng with vertex.xml):<element name="vertex" xmlns="http://relaxng.org/ns/structure/1.0"

datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">

<list> <data type="float"/> <data type="float"/> <data type="float"/> </list></element>

Compact syntax (vertex.rnc also with vertex.xml):element vertex {list {xsd:float, xsd:float, xsd:float}}


Interleave

In RELAX NG, you can define a pattern where element may appear in any order with <interleave>

Restores SGML’s & connector (both in either order) Uses a non-deterministic content model, which DTDs

and XML Schema forbid Can come close with ANY in DTDs and <xs:all>

or <xs:choice> in XML Schema Can use occurrence constraints in <interleave>


Interleave Examples XML syntax (name.rng with name.xml)<element name="name" xmlns="http://relaxng.org/ns/structure/1.0">

<interleave> <element name="family"><text/></element> <oneOrMore> <element name="given"><text/></element> </oneOrMore> </interleave></element>

Compact syntax (name.rnc with name.xml):element name { element family {text}& element given{text}+ }


Mixed Content Model

Mixed content models allow child elements to appear in any order and to be mixed with text

In DTDs, the model is (see mixed.dtd):<!ELEMENT name (#PCDATA | family | given)*>

RELAX NG XML syntax uses <mixed> element


More on Mixed Content

<mixed> is syntax sugar for <interleave> with <text/>

Compact syntax uses mixed keyword, comma separated patterns

XML Schema uses the boolean attribute mixed on either <xs:complexType> or <xs:complexContent>


Mixed Examples

XML syntax (mixed.rng with instance mixed.xml):

<element name="name" xmlns="http://relaxng.org/ns/structure/1.0">

<mixed><element name="family"><text/></element><element name="given"><text/></element></mixed>

</element> Compact syntax (mixed.rnc with instance mixed.xml):

element name {mixed { element family {text}, element given{text}}}


Grammars

The <externalRef> element references an external pattern via a required href attribute

You can combine grammars using the combine attribute on define with a value of interleave or &= (match in any order) or choice or |= (match one of any)

The <include> element merges grammar together (has an href attribute)


More on Grammars

You can nest a grammar in an <include> element which will override a definition with the same name

You can also use <notAllowed/> when merging grammars, forcing you to redefine not-allowed grammars

Grammars can be nested; <parentRef> references or escapes to the parent grammar in a nested grammar, allowing you to redefine a child grammar using a parent, a sort of import


Name Classes

Name classes allow you to include or exclude whole classes of names in a pattern

<name> allows a given name <anyName/> allows any name <nsName> allows names from a given

namespace The <except> element removes names

from a class


Schematron & RELAX NG

Schematron is an assertion-based, rather than a grammar-based schema language

Uses path expressions (XSLT and XPath) The “feather duster” that can reach corners

of your instances, where grammars can’t (Rick Jelliffe)

Co-occurrence constraints possible Can embed Schematron in RELAX NG


Annotations

You can use elements and attributes from other vocabularies in RELAX NG, such as XHTML

<a:documentation> is a special documentation element defined in the RELAX NG DTD compatibility spec (http://www.oasis-open.org/committees/relax-ng/compatibility.html)

<div> allows a place to add foreign attributes for documentation purposes


RELAX NG Resources

RELAX NG: http://www.relaxng.org OASIS: http://www.oasis-open.org RELAX: http://www.xml.gr.jp/relax/ James Clark, TREX, Jing, Trang:

http://www.thaiopensource.com Design of RELAX NG:

http://www.thaiopensource.com/relaxng/design.html

Conclusion

Thanks for your time!

Comments or questions?

[email protected]

Relax NG, a Schema Language for XML

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