Trang

Translator for RELAX NG Schemas

Copyright © 2002 Thai Open Source Software Center Ltd

See the file copying.txt for copying permission.

Version 2002-07-24

Trang translates schemas written in RELAX NG into different formats. In particular, it can

• translate a RELAX NG schema in the compact syntax into the XML syntax
• translate a RELAX NG schema in either the XML or compact syntax into a DTD

Trang aims to produce human-understandable schemas; it tries for a translation that preserves all aspects of the input schema that may be significant to a human reader, including the definitions, the way the schema is divided into files, annotations and comments.

The implementation is available for download as

• a JAR file
• source code (this uses the Jing source code)

The main class is com.thaiopensource.relaxng.output.Driver.

Trang requires at least two command-line arguments: the first is the URI or filename of the schema to be translated; the second is the output filename.

Trang infers the input and output format from the extension of input and output filenames as follows:

.rng

RELAX NG (XML syntax)

.rnc

RELAX NG compact syntax

.dtd

XML 1.0 DTD

.xsd

W3C XML Schema

This inference can be overridden using the -i and -o options.

Trang accepts the following command-line options:

-i format

format specifies the input format as follows:

rng

RELAX NG (XML syntax)

rnc

RELAX NG compact syntax

-o format

format specifies the output format as follows:

rng

RELAX NG (XML syntax)

dtd

XML 1.0 DTD

.xsd

W3C XML Schema

-e enc

Use the encoding enc for schema input and output.

DTD output

The DTD output module of Trang has many limitations. There are many RELAX NG features that it cannot handle, including:

• Wildcards
• Multiple element patterns with the same name
• externalRef
• overriding definitions (in an include)
• combining definitions with combine="choice"

However, Trang can handle many RELAX NG features, including some that go beyond the capabilities of DTDs. When some part of a RELAX NG schema cannot be represented exactly in DTD, Trang will try to approximate it. The approximation will always be more general, that is, the DTD will allow everything that is allowed by the RELAX NG schema, but there may be some things that are allowed by the DTD that are not allowed by the RELAX NG schema. For example, if the RELAX NG schema specifies that the content of an element is a string conforming to some datatype, then Trang will make the content of the element be (#PCDATA); or if the RELAX NG schema specifies a choice between two attributes x and y, then the DTD will allow both x and y optionally. Whenever Trang approximates, it will give a warning message.

If you want to be able to generate a DTD but need to use some feature of RELAX NG that Trang is unable to convert into a DTD, then you might try one of the following approaches:

• Create a RELAX NG schema including the features you need, and then use XSLT (or some other XML transformation language) to transform the schema into something that Trang can handle, perhaps making use of annotations in the schema to guide the transformation.
• Create a RELAX NG schema S₁ which uses only features that Trang can handle but which, consequently, does not capture all the desired constraints; then create a second RELAX NG schema S₂ that includes S₁, and overrides definitions in S₁ replacing them with definitions that make unrestricted use of the features of RELAX NG.

W3C XML Schema

This is still under active development. It currently has several limitations including:

• it may generate schemas that violate W3C XML Schema's restrictions on ambiguous content models;
• it may generate schemas that violate W3C XML Schema's restrictions on consistent element types;
• when the RELAX NG schema cannot be represented by W3C XML Schema, a generalization is generated; it should give a warning in this case, but does not always do so.

Future plans

Trang is constructed around a RELAX NG object model designed to support schema conversion. Trang can support additional input and output formats simply by adding modules that convert these formats from and to this object model.

Features planned for future releases include:

• Provide an output module for the RELAX NG compact syntax
• Enhance the DTD output module to support more features of RELAX NG
• Adapt DTDinst for use as a DTD input module

Related software

DTDinst translates from DTDs into RELAX NG; like Trang, it tries for a translation that preserves definitions and file structure. Trang can handle all the features in RELAX NG schemas generated by DTDinst.

The Sun RELAX NG Converter converts from a variety of different schema languages into RELAX NG. However, it is based on an object model which is designed to support validation, and so does not preserve aspects of the input schemas that are irrelevant for validation (such as the definitional structure).