XML Reference Guide

Mar 14, 2003

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There's no rule that says that you absolutely have to use DTDs, or even W3C XML Schemas to define a grammar for your XML documents. One alternative schema proposal is RELAX NG, pronounced "relaxing". A combination of the original RELAX specification and the TREX specification, RELAX NG provides a simpler way of defining XML grammars than using W3C XML Schemas.

Consider our original document:

<order orderid="THX1138" customerNumber="3263827">
    <lineitem itemid="C33">
       <quantity>36</quantity>
       <unitprice currency="dollars">.35</unitprice>
    </lineitem>
    <lineitem itemid="M48">
       <quantity>1</quantity>
       <unitprice>2200</unitprice>
    </lineitem>
</order>

It's a single root order element, with one or more lineitem children. The lineitems each have a single quantity and unitprice element. We can express the element structure in a simple, almost natural structure in RELAX NG:

<element name="order" xmlns="http://relaxng.org/ns/structure/1.0"> 
    <oneOrMore> 
        <element name="lineitem"> 
             <element name="quantity"> 
                   <text/> 
             </element> 
             <element name="unitprice"> 
                   <text/> 
            </element> 
        </element> 
    </oneOrMore> 
</element>

First, notice that because we can have more than one lineitem element, we've specified it as oneOrMore. If lineitem were optional, we'd specify at as zeroOrMore. Without a modifier, we've specified that quantity and unitprice must appear once and only once.

Next, notice that the quantity and unitprice elements can have text content. RELAX NG enables you to specify all sorts of choices, but for a simple element it's typically text or empty, as in

<element name="status">
    <empty/>
<element>

RELAX NG also allows you to easily specify a particular range of values, which we'll see in a moment with the currency attribute.

Adding attributes is also simple:

<element name="order" xmlns="http://relaxng.org/ns/structure/1.0"> 
    <attribute name="orderid"><text /></attribute>
    <attribute name="customerNumber"><text /></attribute>
    <oneOrMore> 
        <element name="lineitem"> 
             <attribute name="itemid"><text /></attribute>
             <element name="quantity"> 
                   <text/> 
             </element> 
             <element name="unitprice"> 
                   <attribute name="currency"><text /></attribute>
                   <text/> 
            </element> 
        </element> 
    </oneOrMore> 
</element>

As we did with elements, we can specify that attributes can have arbitrary text values, as we're doing here, but we also have the option to specify particular values. For example:

...
     <element name="unitprice"> 
          <attribute name="currency">
               <choice>
                   <value>dollars</value>
                   <value>pounds</value>
               </choice>
          </attribute>
          <text/> 
     </element> 
...

In this case, we can specify a value of either dollars or pounds. This nice thing about this syntax, however, is that we can also use it for elements.

We can also easily specify that an attribute or an element is optional. For example, we might make the currency attribute optional:

...
     <element name="unitprice"> 
          <optional>
               <attribute name="currency">
                   <choice>
                       <value>dollars</value>
                       <value>pounds</value>
                   </choice>
               </attribute>
          </optional>
          <text/> 
     </element> 
...

This ability to specify optional elements also makes it easy to specify mixed content, in which elements and plain text appear together as the content of an element. For example, we can define a mixed content description element that may include the product name and price:

...
    <element name="description">
        <mixed>
            <optional>
                <element name="productname">
                    <text />
                </element>
            </optional>
            <optional>
                <element name="unitprice">
                    <optional>
                        <attribute name="currency">
                            <choice>
                                <value>dollars</value>
                                <value>pounds</value>
                            </choice>
                        </attribute>
                    </optional>
                    <text/> 
                </element>
            </optional>
        </mixed>
    </element>
...

You may have noticed that we now have a duplicate definition for the unitprice element. Like XML Schemas, RELAX NG enables us to define various element and attribute types and then refer to them within the document. For example, we can describe the structure of the document as:

<grammar xmlns="http://relaxng.org/ns/structure/1.0"> 
<start>
    <element name="order"> 
        <attribute name="orderid"><text /></attribute>
        <attribute name="customerNumber"><text /></attribute>
        <ref name="orderContent" />
    </element>
</start>

<define name="orderContent">
    <oneOrMore> 
        <element name="lineitem"> 
             <attribute name="itemid"><text /></attribute>
             <ref name="lineitemContent" /> 
        </element> 
    </oneOrMore> 
</define>

<define name="lineitemContent">
    <element name="quantity"> 
        <text/> 
    </element> 
    <element name="unitprice"> 
        <optional>
            <attribute name="currency"><text /></attribute>
        </optional>
        <text/> 
    </element>
    <optional>
        <element name="description">
            <ref name="descriptionContent"/>
        </element>
    </optional>
</define>

<define name="descriptionContent">
    <mixed>
        <optional>
            <element name="productname">
                <text />
            </element>
        </optional>
        <optional>
            <element name="unitprice">
                <optional>
                    <attribute name="currency">
                        <choice>
                            <value>dollars</value>
                            <value>pounds</value>
                        </choice>
                    </attribute>
                </optional>
                <text/> 
            </element>
        </optional>
    </mixed>
</define>
</grammar>

We define a grammar and definitions, with the start element showing where the actual structure definition starts.

RELAX NG is intended to be more simple to use than other methods of validation such as XML Schemas, so it doesn't include some of the more data-related functionality such as referential entigrity, in which the value of one element or attribute must be something that's been defined elsewhere in the document. It also leaves out support for the definition of entities and notations. Still, it's got a lot of power, particularly when combined with related specifications such as RELAX NG DTD Compatibility. For more power, you can even reference XML Schema datatypes.

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