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<TITLE>XML Canonical Forms</TITLE>
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<H1>XML Canonical Forms</H1>
<P><FONT COLOR=RED><b><em>DRAFT 1</em></b></FONT>
<P> As with many sorts of structured information, there are many
categories of information that may be deemed "important" for
some task. Canonical forms are standard ways to represent
such classes of information. For testing XML, and potentially
for other purposes, three <em>XML Canonical Forms</em> have
been defined as of this writing: <UL>
<LI> <a href=#cxml1>First XML Canonical Form</a>, defined by
James Clark, is also called <em>Canonical XML</em>.
<LI> <a href=#cxml2>Second XML Canonical Form</a>, defined
by Sun, supports testing a larger subset of the XML 1.0
processor requirements by exposing notation declarations.
<LI> <a href=#cxml3>Third XML Canonical Form</a>, defined
by Sun, extends the second form to reflect information
which validating XML 1.0 processors are required to report.
</UL>
<P> For a document already in a given canonical form, recanonicalizing
to that same form will change nothing. Canonicalizing second or
third forms to the first canonical form discards all declarations.
Canonicalizing second or third forms to the other form has no effect.
<P> <em>The author is pleased to acknowledge help from
James Clark in defining the additional canonical forms.</em>
<A NAME=cxml1>
<H2>First XML Canonical Form</H2>
</A>
<P> <em>This description has been extracted from the version at
<a href=http://www.jclark.com/xml/canonxml.html>
http://www.jclark.com/xml/canonxml.html</a>.</em>
<P>
Every well-formed XML document has a unique structurally equivalent
canonical XML document. Two structurally equivalent XML
documents have a byte-for-byte identical canonical XML document.
Canonicalizing an XML document requires only information that an XML
processor is required to make available to an application.
<P>
A canonical XML document conforms to the following grammar:
<PRE>
CanonXML ::= Pi* element Pi*
element ::= Stag (Datachar | Pi | element)* Etag
Stag ::= '<' Name Atts '>'
Etag ::= '</' Name '>'
Pi ::= '<?' Name ' ' (((Char - S) Char*)? - (Char* '?>' Char*)) '?>'
Atts ::= (' ' Name '=' '"' Datachar* '"')*
Datachar ::= '&amp;' | '&lt;' | '&gt;' | '&quot;'
| '&#9;'| '&#10;'| '&#13;'
| (Char - ('&' | '<' | '>' | '"' | #x9 | #xA | #xD))
Name ::= (see XML spec)
Char ::= (see XML spec)
S ::= (see XML spec)
</PRE>
<P>
Attributes are in lexicographical order (in Unicode bit order).
<P>
A canonical XML document is encoded in UTF-8.
<P>
Ignorable white space is considered significant and is treated equivalently
to data.
<A NAME=cxml2>
<H2>Second XML Canonical Form</H2>
</A>
<P><FONT COLOR=RED><b><em>Modified to ensure that literals are surrounded by single quotes.</em></b></FONT>
<P> This canonical form is identical to the first form, with
one significant addition. All XML processors are required to
report the name and external identifiers of notations that
are declared and referred to in an XML document (section 4.7);
those reports are reflected in declarations in this form,
presented in lexicographic order.
<P> Note that all public identifiers must be normalized before being
presented to applications (section 4.2.2).
<P> System identifiers are normalized on output to be relative
to the input document, if that is possible, with the shortest
such relative URI. All other URIs must be absolute. Any
hash mark and fragment ID, if erroneously present on input, are
removed. Any non-ASCII characters in the URI must be escaped
as specified in the XML specification (section 4.2.2).
<PRE>
CanonXML2 ::= DTD2? CanonXML
DTD2 ::= '<!DOCTYPE ' name ' [' #xA Notations? ']>' #xA
Notations ::= ( '<!NOTATION ' Name '
(('PUBLIC ' PubidLiteral ' ' SystemLiteral)
|('PUBLIC ' PubidLiteral)
|('SYSTEM ' SystemLiteral))
'>' #xA )*
PubidLiteral ::= "'" PubidChar* "'"
SystemLiteral ::= "'" [^']* "'"
</PRE>
<P> The requirement of this canonical form differs slightly from that
of the XML specification itself in that all declared notations
must be listed, not just those which were referred to.
<em>Should that change? SAX supports it easily.</em>
<A NAME=cxml3>
<H2>Third XML Canonical Form</H2>
</A>
<P> This canonical form is identical to the second form, with
two significant exceptions reflecting requirements placed on
validating XML processors:<UL>
<LI> They are required to report "white space appearing in
element content" (section 2.10). Ignorable whitespace is
not represented in this canonical form.
<LI> They must report the external identifiers and notation name
for unparsed entities appearing as attribute values (section 4.4.6).
Such entities are declared in this canonical form, in lexicographic
order.
</UL>
<P> This builds on the grammar productions included above.
<PRE>
CanonXML3 ::= DTD3? CanonXML
DTD3 ::= '<!DOCTYPE ' name ' [' #xA Notations? Unparsed? ']>' #xA
Unparsed ::= ( '<!ENTITY ' Name '
(('PUBLIC ' PubidLiteral ' ' SystemLiteral)
|('SYSTEM ' SystemLiteral))
'NDATA ' Name
'>' #xA )*
</PRE>
<P> The requirement of this canonical form differs slightly from that
of the XML specification itself in that all declared unparsed entities
must be listed, not just those which were referred to.
<em>Should that change? SAX supports it easily.</em>
<P>
<ADDRESS>
<A HREF="mailto:[email protected]">[email protected]</A>
</ADDRESS>
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