[NNTP] wildmat-exact

[Clive D.W. Feather]

Peter Robinson said:
> As will any implementor coming across this for the first time.  In any
> case, it's (now) just a comment in the ABNF so it has no power to
> disambiguate anything.

I thought I'd seen - elsewhere - comments being used to provide normative
but hard-to-describe-syntactically restrictions. However, I can't find
anything in RFC2234 on the topic.

See also Charles's comment about Van Wijngaarden grammars.

> Is there a good reason
> why we don't have the following: ?
> 
>         wildmat = ["!"] wildmat-pattern *("," ["!"] wildmat-pattern)
>                   ^^^^^

A wildmat of the form "!a" would mean, according to our semantics, either
the same as "*,!a" or would match nothing. The former is better expressed
explicitly and the latter is nugatory.

> The other possibility is to exclude ! from wildmat-exact.  Any future
> extension that needs to use wildmats to match strings containing ! will
> probably need to be able to match the other excluded characters (*?,\[])
> as well as "!".  Presumably it will do that by permitting them and using
> \ as an escape character.

Which is something we looked out but left out, for now, because of problems
with the corner cases.

> These are both independent of whether we change the overall wildmat ABNF
> to allow it to start with a "!", meaning not.

Which I am against.

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