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Re: Non-deterministic content model
- From: Joe English <email@example.com>
- To: firstname.lastname@example.org
- Date: Thu, 14 Jun 2001 09:30:27 -0700
Arthur Rother wrote:
> I came across this one in my first lessons on learning DTD's, where I tried
> to write a DTD for a game of chess:
> ((whitemove, blackmove)*, whitemove?)
That's a neat example. It's well-known that ((a,b)*,a?)
is hopelessly ambiguous (i.e., there's no way to write
it as deterministic content model), but I've never
seen a real-world example where that construct was
useful before now.
> If this is the only endless case, one could hardcode this in the parser.
It's not -- there's an infinite number of "hopelessly ambiguous"
> How do [XML Spy and XMetal] deal with it?
There are plenty of algorithms for regular expression
matching besides the one prescribed for SGML.
You can convert to a DFA (lex), backtrack (Perl),
use derivatives (TREX), or simulate an NDFA,
to name a few.
The only real problem with nondeterministic
content models in SGML has to do with start-tag
inference (and even that can be worked around).
Since XML doesn't support this feature, it's
not as big of a problem for implementations.