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.
Related Links
"The Design of RELAX NG," a paper by James Clark
RELAX NG 1.0 DTD compatibility specification
RELAX NG compact syntax specification
Jing, James Clark's RELAX NG processor (Java)
Trang, James Clark's RELAX NG compact syntax processor (Java)
Multi-schema Validator (MSV), Sun's schema validator (by Kawaguchi Kohsuke)
Murata Makoto's online RELAX NG validator (Java/JSP)
Eric van der Vlist's online RELAX NG validator (Python)
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Comment on this Article
| Titles Only | Titles Only | Newest First |
- Something wrong
2006-10-27 08:34:49 johnhmichael [Reply]
The tutorial about RELAX NG's compact syntax and default value reads something like:
[ a:defaultValue = "bla-bla" ]
{ attribute a0 { huhu }}
It seems to me that a question mark should close this expression:
[ ... ] attribute a0 { ... }}?
Anyway, the "jing" valider signals an error if it not the case.
All the best,
johnhmichael
- So what?
2002-06-21 09:46:58 Paul Strand [Reply]
I thought the whole point of defining something other than DTD's (Relax NG or Schema, for example) was to get away from the syntax of DTD's which is not as readily parseable.
If XML is described by XML, you only need one parser at your side. Unfortunately, W3C Schemas are so complicated that the advantage of learning only one syntax is offset by the inability to quickly read that syntax for meaning. My take on this was that Relax NG was the compromise. However, they just reinvented DTD's by deriving an XML syntax based off of the DTD syntax and then derived a completely new syntax from that.
If they were going to go to all that trouble, why didn't they just use "micro processing" to simplify the syntax of DTD's into something easily readible, yet XML parseable? For example (and forgive the gratuitous use of Polish notation), they could have done something like this:
<!ELEMENT DTD (ELEMENT+)>
<!ATTLIST DTD
root NMTOKEN #IMPLIED>
<!ELEMENT ELEMENT (ATTRIBUTE*)>
<!ATTLIST ELEMENT
name NMTOKEN #REQUIRED
sequence CDATA #IMPLIED
content (PCDATA | EMPTY | ANY) #IMPLIED>
<!ELEMENT ATTRIBUTE EMPTY>
<!ATTLIST ATTRIBUTE
name NMTOKEN #REQUIRED
type (CDATA | NMTOKEN | NMTOKENS | ID | IDREF | IDREFS) #IMPLIED
enumeration CDATA #IMPLIED
declaration (IMPLIED | REQUIRED | FIXED) #IMPLIED
default CDATA #IMPLIED>
...which defines a DTD-similar syntax, reusing "*", "?", "|", "+" and ",", from DTD's as operators. The above is the DTD which defines how that syntax would be used. Below is how it would be expressed in its own syntax:
<!DOCTYPE DTD SYSTEM "XMLDTD.dtd">
<DTD root="DTD">
<ELEMENT name="DTD" sequence="+ELEMENT">
<ATTRIBUTE name="root" type="NMTOKEN" declaration="IMPLIED"/>
</ELEMENT>
<ELEMENT name="ELEMENT" sequence="*ATTRIBUTE">
<ATTRIBUTE name="name" type="NMTOKEN" declaration="REQUIRED"/>
<ATTRIBUTE name="sequence" type="CDATA" declaration="IMPLIED"/>
<ATTRIBUTE name="content" enumeration="| PCDATA | EMPTY ANY" declaration="IMPLIED"/>
</ELEMENT>
<ELEMENT name="ATTRIBUTE" content="EMPTY">
<ATTRIBUTE name="name" type="NMTOKEN" declaration="REQUIRED"/>
<ATTRIBUTE name="type" enumeration="| CDATA | NMTOKEN | NMTOKENS | ID | IDREF IDREFS" declaration="IMPLIED"/>
<ATTRIBUTE name="enumeration" type="CDATA" declaration="IMPLIED"/>
<ATTRIBUTE name="declaration" enumeration="| IMPLIED | REQUIRED FIXED" declaration="IMPLIED"/>
<ATTRIBUTE name="default" type="CDATA" declaration="IMPLIED"/>
</ELEMENT>
</DTD>
(For those of you not familiar with Polish notation, the operator comes before the one or two operands. So, A + B in Polish notation becomes + A B. This removes the necessity for parenthesis and makes parsing in this case easier.)
So, what does this compact syntax gain me? Actually, why go with Relax NG at all if it is not a Recommendation and it suffers from the pitfalls of both DTD's and Schema?
