Extension

The XML Catalog format is extensible. For example, an “suffix rewriting” extension:

<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"
         xmlns:sfx="http://nwalsh.com/xcatalog/1.0">
  <sfx:systemSuffix suffix="docbookx.dtd"
       uri="/share/doctypes/docbook42/xml/docbookx.dtd"/>
</catalog>

This extension replaces any system identifier that ends in “docbookx.dtd” with the specified URI.