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Re: [abnf-discuss] ABNF/RFC7405/ Update: EBNF syntactic exception

John Levine <johnl@taugh.com> Fri, 08 July 2022 17:04 UTC

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From: John Levine <johnl@taugh.com>
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Subject: Re: [abnf-discuss] ABNF/RFC7405/ Update: EBNF syntactic exception
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It appears that Tony Finch  <dot@dotat.at> said:
>ABNF at the moment describes context-free grammars, which means there are
>lots of options for turning an ABNF grammar into a parser. But CFGs are
>not closed under complement or intersection, so if you add an unrestricted
>negation operator, you are changing the class of languages that the
>grammar can represent.

This is a deal-breaker.  While I agree that a negation operator could be nice,
it's not nice enough to break all of the ABNF tools we have.

Previous extensions like case sensitivity needed only a tweak to the code
of ABNF verifiers, but this would need a complete rewrite.

R's,
John

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