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RELAX NG's Compact Syntax
by Michael Fitzgerald | Pages: 1, 2, 3

Going Context Free

I mentioned earlier in the article that the compact syntax can look like a context-free grammar. The following example uses a start symbol and other symbols that serve as terminals and non-terminals. For example, the symbol year, on the left side of the equals sign, may be considered a non-terminal, and the element definition on right side, a terminal:


# RELAX NG schema for a date

start = date

date = element date { attribute type { text },
 (year & month & day), limits*}

year = element year { text }
month = element month { text }
day = element day { text }

include "limits.rnc"

The following instance is valid with regard to the foregoing:


<?xml version="1.0" encoding="UTF-8"?>
<date type="US">
 <month>June</month>
 <day>1</day>
 <year>2002</year>
 <limits days="30"/>
</date>

When translated, this example creates a different RELAX NG schema than the ones shown previously, producing a <grammar> and <start> element and several <define> elements, as seen in this incomplete fragment:


<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <!-- RELAX NG schema for a date -->
  <start>
    <ref name="date"/>
  </start>
  <define name="date">
    <element name="date">
      <attribute name="type"/>
      <interleave>
        <ref name="year"/>
        <ref name="month"/>
        <ref name="day"/>
      </interleave>
      <zeroOrMore>
        <ref name="limits"/>
      </zeroOrMore>
    </element>
  </define>

A <grammar> element is a container for definitions. The <start> element indicates the document element for an instance, just as a document type declaration does. The <define> elements contain patterns which can be referenced by name (with a <ref> element) and therefore easily reused.

Back in the compact schema, a symbol for the limits pattern (modified with *) was added to the end of the content model for date, but where is it defined? It's defined in the included schema limits.rnc (see the last line of the last compact example), which looks like


# Limits for year, months, and days
limits =
 element limits {
  attribute years { text }?,
  attribute months { text }?,
  attribute days { text }?
 }

When processed, the included compact schema is translated into RELAX NG XML syntax as well. The resulting filename, limits.rng, is inferred from limits.rnc. The included pattern contains a definition for the limits element, which may contain up to three optional attributes. The absence of element children indicates that its content is empty.

Conclusion

It would several more articles to cover all aspects of RELAX NG in fair detail. This article has only touched lightly on its compact syntax and some of the more commonly used structures of the language. I have neglected some interesting things: for example, lists, name classes, merging grammars, and combining definitions. If you've gotten behind the wheel and tested these examples for yourself, you likely have a good feel for just how easy RELAX NG's compact syntax is to learn and use.

Going Context Free

Conclusion

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