> Is it correct, that this is not a legal XML file (since an external
> entity &message;

well &message; isn't defined so you can't tell if it is internal
(defined by
<!ENTITY message "hello">
)
or
external, defined by
<!ENTITY message SYSTEM "foo.xml">




> These productions imply that doctypedecl is optional.

yes but that is so that
<x>foo</x> 
is well formed without a doctypedecl. You need it if you need it.

> Can somebody please point to the
> relevant section of the XML spec, where the above XML proves to be
> illegal.

http://www.w3.org/TR/xml/#wf-entdeclared

...  the Name given in the entity reference MUST  match that in an
entity declaration that ...


> Can I anyhow successfully parse the above XML? for e.g., using a
> custom entity resolver, or using a catalog file.

you can use a catalog file that defaults in a dtd that defines the
entity.

Actually if you swap in _any_ external DTD and use a non validating
parser you may (depending on configuration, but not for example in
xpath) be able to parse past the syntax as an undefined entity ref.
because of the paragraph following the one quoted above which says

Note that non-validating processors are not obligated to read and
process entity declarations occurring in parameter entities or in the
external subset; for such documents, the rule that an entity must be
declared is a well-formedness constraint only if standalone='yes'. 

David

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