Xml Matters #26: More On Relax Ng

Tools and Special Issues

David Mertz, Ph.D.
Facilitator, Gnosis Software, Inc.
January 2003

RELAX NG Schemas provide a more powerful, more concise, and semantically more straightforward means of describing classes of valid XML instances than do W3C XML Schemas. This installment continues the discussion of RELAX NG begun in the last one, addresses a few addition semantic issues, and looks at tools for working with RELAX NG.

More Semantic Issues

The last installment gave readers a fairly complete overview of both the syntax and semantics of RELAX NG schemas. However, a few issues were glossed over, and are worth touching on.

Both DTDs and W3C XML Schemas allow for infoset augmentation, while RELAX NG does not. James Clark--one of the creators of RELAX NG (and of many widely used XML tools)--argues vehemently that infoset augmentation violates modularity in the roles of XML instance documents and schemata. In other words, for Clark, RELAX NG has a feature where DTDs and W3C Schemas have a bug. My own feelings on the matter are mixed, but I can get his intuition.

Let us backtrack a little and explain what this infoset stuff is about. Basically, we can ask of an XML instance what data it contains. If we parse the instance without validation, the answer depends on nothing other than what values occur in its attributes and element bodies. If we value modularity, a schema should only tell us whether an instance is valid or not, it should not change that actually information in a document. However, such modularity is violated if we use a DTD or W3C XML Schema for validation. For example, consider the following DTD:


<!ATTLIST foo bar CDATA "curious"
              baz CDATA #FIXED "curiouser">

And the XML instance:


<?xml version="1.0"?>
<!DOCTYPE foo SYSTEM "curious.dtd">

A non-validating parser finds a different set of information in this document than a validating parser. Contrast the non-validating utility xmlcat with the validating 4xml (both echo whatever they "see" back to the console):

% ./xmlcat curious.xml
<?xml version="1.0" encoding="iso-8859-1"?>
% 4xml -p curious.xml
<?xml version="1.0" encoding="utf-8"?>
<foo bar="curious" baz="curiouser"/>

An W3C Schema allows default and fixed attributes to have similar effects for both <xsd:attribute> and <xsd:element> tags.

The argument in favor of defaulting is that it allows XML instance minimization. I have used defaults (or more likely #FIXED attributes) for this very purpose. But I can also see dangers--both of malice and of debugging nightmares--in the idea that the very content of a local XML document depends on a remote URI (perhaps spoofable), and even upon an absence of network interruptions during parsing.

In any case, RELAX NG does not peform any infoset augementation. Well, almost; I think Clark overstates this point. If you impose a datatype on an element or attribute, you still change the content of the value in an important way. The value of the string "1.0" is different from the value of the float 1.0, even though the two are represented in exactly the same way in an XML instance.

Stating Cardinality

Basically, W3C XML Schemas have better means of requiring occurrence cardinalities than do DTDs or RELAX NG schemas. If you want a <foo> element to occur between 5 and 30 times within the <bar> element, you can decare this in W3C Schemas with a straightforward:

W3C XML Schema cardinality rule

<xsd:element name="bar">
  <xsd:element name="foo" minOccurs="5" maxOccurs="30"/>

The same cardinality rule can be stated in a DTD, but very clumsily:

DTD cardinality rule

    (foo, foo, foo, foo, foo, foo?,foo?,foo?,foo?,foo?
     foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?) >

What I would like for RELAX NG would be an explicit <cardinality> tag, so that you could (hypothetically) write something like:

Imagined RELAX NG 2.0 cardinality rule

<element name="bar" xmlns="http://relaxng.org/ns/structure/1.0>
  <cardinality min="5" max="30">
    <element name="foo"/>

Unfortunately, in the actually existing RELAX NG, the only cardinalities you get are <zeroOrMore>, <oneOrMore>, and <optional>. However, named patterns can at least be used to make spelling out cardinalities slightly less painful. In compact syntax, for example:

Actual RELAX NG compact syntax cardinality rule

start = element bar { fivefoo, upto25foo }
fivefoo = element foo { empty }, element foo { empty },
          element foo { empty }, element foo { empty },
          element foo { empty }
maybefoo = element foo { empty }?
upto25foo =
  fivefoo?, fivefoo?, fivefoo?, fivefoo?,
  maybefoo, maybefoo, maybefoo, maybefoo, maybefoo

I confess that this sort of naming is not perfect, but at least it is possible to name large numbers by effectively raising to powers though repetition of named patterns.

Transformations And Validations

There are a variety of tools available for working with RELAX NG schemas. Java is the language in which these tools are predominantly implemented, but some tools/libraries have been written in in Python, C#, Visual Basic. Surprisingly, I have not found any libraries written in other languages that would seem to be good fits: Perl, Ruby, C/C++.

One obvious class of RELAX NG application is validators. Just as with validating parsers that work with DTDs or W3C XML Schemas, a number of command-line, online, or library parsers are available for RELAX NG. A slightly less obvious class of application is tools to transform schemata into each other. Sun's RELAX NG Converter and James Clark's trang and DTDinst let you convert among RELAX NG (XML and compact syntax), DTDs, and W3C XML Schemas. I plan to write a less ambitious Python tool compact2xml.py in time for the next installment, which will allow 4Suite and xvif to utilize RELAX NG compact syntax (both authors have expressed an interest in including such a tool).

It is worth looking at tranformations in a bit more detail. The first installment in this sequence looked at ways in which RELAX NG is strictly more powerful than W3C XML Schemas, and looking at some "best effort" transformations help illustrate this point. For example, the previous installment presented a schema for a library patron, which is expressed in compact syntax as:

#-------------- Libary Patron Compact Syntax--------------#
element patron {
  element name { text }   &
  element id-num { text } &
  element book {
    attribute isbn { text } |
    attribute title { text }

See the first part for the XML syntax version, which is semantically identical, albeit more verbose. trang make a good effort at turning this into a W3C XML Schema. The file extensions of the input and output file are used to guess types (or may be overridden with switches):

Transforming RELAX NG to W3C XML Schema

% java -jar trang.jar patron.rnc patron.xsd
% cat patron.xsd
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified" version="1.0">
  <xsd:element name="patron">
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="name"/>
        <xsd:element ref="id-num"/>
        <xsd:element ref="book"/>
  <xsd:element name="name">
    <xsd:complexType mixed="true"/>
  <xsd:element name="id-num">
    <xsd:complexType mixed="true"/>
  <xsd:element name="book">
      <xsd:attribute name="isbn"/>
      <xsd:attribute name="title"/>

To the credit of trang, I think this W3C Schema is genuinely the best that can be done for the situation. Every XML instance accepted by the RELAX NG schema is also accepted by the W3C XML Schema, and many errors are rejected by both. The problem is, that there is a distinct class of XML instances that are not really valid according to our desired rule, but that pass validation with the W3C Schema. For example:

Limits of W3C XML Schema discernment

% cat patron-i1.xml
<?xml version="1.0" encoding="UTF-8"?>
  <book isbn="0-528-84460-X"/>
  <name>John Doe</name>    <!-- repeats name subelement -->
  <name>Second Name</name>
  <book title="Why RELAX is Clever"/>
% cat patron-i2.xml
<?xml version="1.0" encoding="UTF-8"?>
  <name>John Doe</name>
  <!-- Too many and too few attributes of book element -->
  <book title="Why RELAX is Clever" isbn="0-528-84460-X"/>
% cat patron-i3.xml
<?xml version="1.0" encoding="UTF-8"?>
<patron/>        <!-- No required subelements -->

Of course, even though the above three examples falsely validate, W3C XML Schema will still reject XML instances with entirely disallowed elements/attributes, or ones that nest elements in improper ways (e.g. <book> inside <name> rather than as a sibling).

As far as validation tools go, I find that jing does a good job of producing useful error messages when validation fails. The Python XML library 4Suite encorporates a version of the xvif library, and also performs validation (the latter is also accessible online, see Resources). But compare the errors:

Validation error messages with jing

% java -jar ../trang/jing.jar patron.rng patron-i3.xml
Error at URL "file:/.../patron-i3.xml",
line number 2: unfinished element
% java -jar ../trang/jing.jar patron.rng patron-i1.xml
Error at URL "file:/.../patron-i1.xml",
line number 5: element "name" not allowed in this context

% 4xml --rng=patron.rng patron-i1.xml
Traceback (most recent call last):
  File "/.../site-packages/Ft/Xml/_4xml.py", line 89, in Run
    raise RngInvalid(result)
Ft.Xml.Xvif.RngInvalid: Qname {None}name not exected
% 4xml --rng=patron.rng patron-i3.xml
Traceback (most recent call last):

Of course, in an application context, the choice of the programming language that will utilize the libraries outweighs differences in produced messages.

Compiled Validators

A category of tool that I have not seen much of outside of RELAX NG contexts is a single-schema validator. Take a look at the RELAX NG home page for links to such tools, including Bali, RelaxNGCC. These frameworks will automatically emit code for specialized validation of a particular RELAX NG schema. Presumably, such a specialized validator will run faster than a general purpose one. The reason such tools are possible--or at least much more straightforward than the same thing would be relative to W3C XML Schemas--is because the design of RELAX NG is extremely well grounded in algorithmic analysis.

Relax Ng Enhanced Xml Editors

Unfortunately, XML editors do not yet support RELAX NG as widely as they do W3C XML Schemas. Of course, DTDs remain much more widely supported than either of other schema style. A shame in this is that it would actually be far easier to include customizations around RELAX NG in an editor because of the simple conceptual framework of RELAX NG validation. Ideally, a custom XML editor would utilize a RELAX NG schema to direct and assist a user in the insertion of attributes and elements in ways that maintain validity.

A compromise for the moment could be to use a tool like trang to convert a RELAX NG schema into a W3C XML Schema or DTD that approximates it, then use those within a GUI XML editor. But doing that would help only to a limited extent.

There is one XML editor built around RELAX NG that is mentioned on the RELAX NG home page. It is called XML Operator, and is a Java based tool. I played with the editor a small amount, and found that it could be potentially useful. But XML Operator falls on the low end of the XML editors I have previously reviewed, implementing just a few features here and there, providing neither the huge array of tools of XML-Spy, or the simple elegance of oXygen. I suppose XML Operator is comparable to EditML Pro or XMLWriter (but XML Operator is Free Software, which is a plus).

Until Next Time

The last two installments have looked at most of the elements of RELAX NG, including a summary of tools for working with it. The next installment will touch briefly on how RELAX NG lets you include external schemas in your schema, and selectively merge the specifications of different schemas. But in main, the final installment of this sequence will look at the RELAX NG compact syntax in some more detail, and explain the exact correspondences between compact syntax and XML syntax.


The home page for RELAX NG is at the below URL. This website contains numerous useful links to links, articles, tools, and so on. Of particular note is the excellent tutorial written by two great luminaries of XML technologies, James Clark and Murata Makoto.


James Clark wrote a discussion of the algorithmic principles behind RELAX NG validation. Interestingly, his sample code is provided in Haskell, which has some advantages I have described in my XML Matters installment on the HaXml library:


An online tool to validate an XML instance document against a RELAX NG shcema is at the following URL. A RELAX NG schema itself is validated during the process, as well. This tool is based on Eric van der Vlist's xvif tool, written in Python:


The above online RELAX NG validator lets you select from a set of test cases, as well as check your own examples. The set of test cases provided are also available (and briefly discussed) at:


The xvif library itself can be downloaded from:


However, 4Suite is a somewhat more polished tool that incorporates xvif for RELAX NG validation. The command-line tool 4xml will validate against both RELAX NG and DTDs, with various options. 4Suite includes many other tools and libraries for working with many XML-related technologies:


For background and comparison, an online tool to validate an XML instance document against a W3C XML Schema can be found at:


Also, an online tool to check W3C XML Schemas against the Approved Recommendation:


The tools trang and jing are complementary tools for transformation between schemata and validation against RELAX NG schemas. The former depends on the latter, but both can be downloaded in a convenient archive from:


You will need to optain an implementation of the Java API for XML Processing (JAXP) to use trang. If you run a Java 1.4 JVM, you are fine already; otherwise, obtain crimson at:


Sun's RELAX NG Converter utilizes Sun's Multi-Schema Validator (MSV) to accomplish the same purpose as trang. See:


DTDinst is a Java tool to convert DTDs into an XML instance document format, including handling of parametric entities:


The DTDinst XML format is of limited utility by itself, since nothing else works with it. However, an XSLT stylesheet is available to transform this format into RELAX NG (with a few caveats). You will need an XSLT tool to utilize this:


A collection of schemata and test XML instance documents for the library patron example discussed in this article can be found at:


About The Author

Picture of Author David Mertz, in his gnomist aspirations, wishes he had coined the observation that the great thing about standards is that there are so many to choose from. But then, he is also fuzzy on OS design. David may be reached at [email protected]; his life pored over athttp://gnosis.cx/publish/. Suggestions and recommendations on this, past, or future, columns are welcomed.