2. Specifying which RELAX NG schema to use for validating a document
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## 2. Specifying which RELAX NG schema to use for validating a document

This section is just a primer. The reference documentation about this topic is really XMLmind XML Editor - Configuration and Deployment.

### 2.1. The `relaxng` configuration element

A document type declaration (`<!DOCTYPE>`) can be used to associate a DTD to a document. Attributes `xsi:schemaLocation`/`xsi:noNamespaceSchemaLocation` can be used to associate W3C XML Schemas to a document. But there is no standard way to associate a RELAX NG schema to a document. Therefore this association must be made using an external specification such as the Namespace Routing Language (NRL).

In the case of XMLmind XML Editor, this external specification is simply a configuration element called `relaxng`.

XHTML example:

    <configuration name="XHTML Strict [RELAX NG]"
      xmlns:html="http://www.w3.org/1999/xhtml"
      xmlns="http://www.xmlmind.com/xmleditor/schema/configuration">
      <include location="xxe-config:schema/ns_xhtml.incl" />

      <detect>
        <rootElementNamespace>http://www.w3.org/1999/xhtml</rootElementNamespace>
      </detect>

      <relaxng location="xxe-config:common/rng/xhtml1/xhtml-strict.rng" />

      <preserveSpace elements="html:pre html:style html:script" />

      <css name="XHTML" location="xhtml_rng.css" />
      <template name="Page" location="page.html" />
    </configuration>

| The `relaxng` configuration element specifies the location of the RELAX NG schema (XML syntax or compact syntax) to which conforms the document being opened.
| Unlike the DTD, `xhtml-strict.rng` does not specify a `preserve` default value for attribute `xml:space` of elements such as `pre`. Therefore, the `preserveSpace` configuration element must be used to specify whitespace-preserving elements.

### 2.2. The `<?xxe-relaxng-schema>` processing instruction

The following processing instruction is a non standard, proprietary, way to associate a document to a RELAX NG schema. Its use should be restricted to testing and other quick and dirty experiments.

DocBook example:

    <?xml version="1.0" encoding="UTF-8" ?>
    <?xxe-relaxng-schema name="-//OASIS//RELAX NG DocBook V4.3//EN"
      location="http://www.docbook.org/rng/4.3/docbook.rnc"
      compactSyntax="true" encoding="US-ASCII" ?>

    <!DOCTYPE article [

    <!ENTITY % sgml.features "IGNORE">
    <!ENTITY % xml.features "INCLUDE">

    <!ENTITY euro "&#x20AC;">

    <!ENTITY % dbcent PUBLIC
    "-//OASIS//ENTITIES DocBook Character Entities V4.3//EN"
    "http://www.oasis-open.org/docbook/xml/4.3/dbcentx.mod">
    %dbcent;

    ]>

    <article>
      <title></title>
      <section>
        <title></title>
        <para></para>
      </section>
    </article>

| The `<?xxe-relaxng-schema>` processing instruction has pseudo-attributes identical to the attributes of the `relaxng` configuration element.
| Using a RELAX NG schema should not prevent you from specifying a document type declaration for character entities.

### 2.3. Sample XXE configurations using RELAX NG schemas

The examples used in this section come from the following XXE configurations:

`Bundled DocBook 5 configuration`

  Allows to create and edit DocBook V5.0 documents conforming to the RELAX NG schema coming from DocBook.org.

Add-on "Customization of the DocBook 5 configuration allowing to edit <xi:include> elements by hand"

  This customization the DocBook 5 configuration allows to edit `xi:include` elements by hand, exactly like any ordinary DocBook element. With the stock DocBook 5 configuration, `xi:include` elements are transcluded, which means that such elements exist only at document open time and at document save time.

  Installing this customization is definitely not recommended for a normal usage of DocBook 5. However, if you need it, you may download it and install it using Options → Install Add-ons.

`Add-on "XHTML 1.0/RELAX NG configuration"`

  Allows to create and edit XHTML 1.0 documents conforming to the modular RELAX NG schema written by James Clark.

  This RELAX NG-based configuration does not conflict with the bundled DTD-based XHTML configuration. If you want to experiment with it, you may download it and install it using Options → Install Add-ons.

  ### Caution

  This configuration should be considered as a work in progress. If you just want to author XHTML documents, do not use this RELAX NG-based configuration. Rather use the bundled, DTD-based, configuration which is much more mature.

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