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xom-interest - Re: [XOM-interest] Bug: XOM's rules for system ids are too stringent

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Subject: XOM API for Processing XML with Java

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  • From: Elliotte Rusty Harold <elharo AT metalab.unc.edu>
  • To: John Cowan <cowan AT ccil.org>
  • Cc: xom-interest AT lists.ibiblio.org
  • Subject: Re: [XOM-interest] Bug: XOM's rules for system ids are too stringent
  • Date: Thu, 21 Feb 2008 09:33:37 -0800

John Cowan wrote:
Currently, XOM throws an error if the system identifier in a DOCTYPE
declaration is not a well-formed URI. But that's too stringent: the
XML Recommendation says that a system identifier has to be *convertible*
to a well-formed URI by escaping inappropriate characters. The ipsissima
verba are in Section 4.2.2 of XML 1.0:

System identifiers (and other XML strings meant to be used as URI
references) may contain characters that, according to [IETF RFC
3986], must be escaped before a URI can be used to retrieve the
referenced resource. The characters to be escaped are the control
characters #x0 to #x1F and #x7F (most of which cannot appear in
XML), space #x20, the delimiters '<' #x3C, '>' #x3E and '"' #x22,
the unwise characters '{' #x7B, '}' #x7D, '|' #x7C, '\' #x5C,
'^' #x5E and '`' #x60, as well as all characters above #x7F.


At first glance, this appears to be another incompatible change introduced as an erratum; this time in the 3rd edition of XML 1.0. In the 2nd edition I find this text instead:

[Definition: The SystemLiteral is called the entity's system identifier. It is a URI reference (as defined in [IETF RFC 2396], updated by [IETF RFC 2732]), meant to be dereferenced to obtain input for the XML processor to construct the entity's replacement text.] It is an error for a fragment identifier (beginning with a # character) to be part of a system identifier. Unless otherwise provided by information outside the scope of this specification (e.g. a special XML element type defined by a particular DTD, or a processing instruction defined by a particular application specification), relative URIs are relative to the location of the resource within which the entity declaration occurs. A URI might thus be relative to the document entity, to the entity containing the external DTD subset, or to some other external parameter entity.

Thus the system identifier is a URI, not merely a string that is converted to a URI. It is an error for a URI to contain an unescaped linefeed, and XOM should throw an exception in that case.

If I misread you and XOM is disallowing a properly escaped linefeed in a URI, then that may be a bug I'll have to look at.

--
Elliotte





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