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Date: Wed, 2 Jan 2013 17:48:18 +0000
From: Tony Finch <dot@dotat.at>
To: Dave Crocker <dcrocker@gmail.com>
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Cc: abnf-discuss@ietf.org, Paul Kyzivat <pkyzivat@alum.mit.edu>
Subject: Re: [abnf-discuss] Some thoughts about how to make ABNF more
 convenient to use
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Dave Crocker <dcrocker@gmail.com> wrote:
> On 1/2/2013 8:24 AM, Tony Finch wrote:
> >
> > Hmm, well I would describe those as sub-languages whereas I thought we
> > were talking about independent languages. But in both cases they can be
> > defined / identified using the transitive closure from the starting rule,
> > without revising ABNF.
>
> I don't think so.  The parsing rules for a date field are quite different than
> for an an address field, in 822.  They share some lexical constructs, of
> course, but I don't find that very significant for the current topic.

The reason I say they are sub-languages is that the starting rule for
messages in RFC 5322 is

   message         =   (fields / obs-fields)
                       [CRLF body]

and from there you can get to various sublanguages via rules like

   orig-date       =   "Date:" date-time CRLF

   sender          =   "Sender:" mailbox CRLF

All the ABNF in RFC 5322 is in the transitive closure of the message rule.

Tony.
-- 
f.anthony.n.finch  <dot@dotat.at>  http://dotat.at/
Forties, Cromarty: East, veering southeast, 4 or 5, occasionally 6 at first.
Rough, becoming slight or moderate. Showers, rain at first. Moderate or good,
occasionally poor at first.
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