xml-dev - Re: Comparing PUBLIC identifiers for equality.

[John Cowan <jcowan@reutershealth.com>]

Alan Kennedy wrote:

>  o Are PUBLIC identifers guaranteed to be unique? For example,
> if the PUBLIC identifier "-//W3C//DTD HTML 4.0 Transitional//EN"
> is used, can different people legally use that identifier to refer
> to different things?

Public identifiers can be anything at all provided no illegal characters
are used, so nothing can be said about them.

However, *formal* public identifiers (FPIs) are defined by an ISO
standard, and always begin with "+//", "ISO" or "-//".
In the first two cases, they can only be defined by the owner (as given
by a registry in the first case, or an ISO standard in the second
case).  The "-//" FPIs are technically private use, but in practice
organizations do not step on one another's FPIs.

The reason that "-//" is so common is that the registry for "+//" is
de facto broken.

>  o Is there a standard structure/grammar which I can use to
> decompose PUBLIC identifiers?

Formal public identifiers have the form "-//orgname//type title//lang"
where orgname is a plain-text organization name, type is "DTD" or various
other things, "title" is a plain-text title, and lang is the ISO 639 code
for the natural language in which the document described by the FPI
is written.  In the case of registered organizations, "-" is replaced by "+".
For FPIs defined directly by ISO standards, it's "ISO nnnnn:yyyy whatever"
or "ISO/IEC nnnnn:yyyy whatever".

> If PUBLIC identifiers are globally unique, and always refer to the
> same thing, then obviously (?) I can just do a simple string
> comparison to see if they are equal.

There is no guarantee that a FPI always refers to the same thing, if
by "same" you mean "byte-by-byte identical".
 
-- 
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