Editing XML schema (section)

==== Advantages over W3C XML Schema ====

While the language of RELAX NG can be written in XML, it also has an equivalent form that is much more like a DTD, but with greater specifying power. This form is known as the compact syntax. Tools can easily convert between these forms with no loss of features or even commenting. Even arbitrary elements specified between RELAX NG XML elements can be converted into the compact form.

RELAX NG provides very strong support for unordered content. That is, it allows the schema to state that a sequence of patterns may appear in any order.

RELAX NG also allows for non-deterministic content models.  What this means is that RELAX NG allows the specification of a sequence like the following:

<syntaxhighlight lang="xml">
<zeroOrMore>
  <ref name="odd" />
  <ref name="even" />
</zeroOrMore>
<optional>
  <ref name="odd" />
</optional>
</syntaxhighlight>

When the validator encounters something that matches the "odd" pattern, it is unknown whether this is the optional last "odd" reference or simply one in the zeroOrMore sequence without looking ahead at the data. RELAX NG allows this kind of specification. W3C XML Schema requires all of its sequences to be fully deterministic, so mechanisms like the above must be either specified in a different way or omitted altogether.

RELAX NG allows attributes to be treated as elements in content models. In particular, this means that one can provide the following:

<syntaxhighlight lang="xml">
<element name="some_element">
  <choice>
    <attribute name="has_name">
      <value>false</value>
    </attribute>
    <group>
      <attribute name="has_name">
        <value>true</value>
      </attribute>
      <element name="name"><text /></element>
    </group>
  </choice>
</element>
</syntaxhighlight>

This block states that the element "some_element" must have an attribute named "has_name". This attribute can only take true or false as values, and if it is true, the first child element of the element must be "name", which stores text. If "name" did not need to be the first element, then the choice could be wrapped in an "interleave" element along with other elements. The order of the specification of attributes in RELAX NG has no meaning, so this block need not be the first block in the element definition.

W3C XML Schema cannot specify such a dependency between the content of an attribute and child elements.

RELAX NG's specification only lists two built-in types (string and token), but it allows for the definition of many more. In theory, the lack of a specific list allows a processor to support data types that are very problem-domain specific.

Most RELAX NG schemas can be algorithmically converted into W3C XML Schemas and even DTDs (except when using RELAX NG features not supported by those languages, as above). The reverse is not true. As such, RELAX NG can be used as a normative version of the schema, and the user can convert it to other forms for tools that do not support RELAX NG.