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From: Richard Stallman <rms@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: Explicitly numbered subgroups in regular expressions
Date: Sun, 10 Jun 2007 09:19:00 -0400
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To: Stefan Monnier <monnier@iro.umontreal.ca>
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In-reply-to: <jwvfy50c28x.fsf-monnier+emacs@gnu.org> (message from Stefan
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Xref: news.gmane.org gmane.emacs.devel:72569
Archived-At: <http://permalink.gmane.org/gmane.emacs.devel/72569>

    What it does is add a new regexp syntax \(?<num>:<regexp>\) which is like
    \(<regexp>\) except that it specifies explicitly the number of the subgroup.
    E.g. (and (string-match "\\(?3:a\\)" "a") (match-data)) returns (0 1 nil
    nil nil nil 0 1).  There is no backward compatibility issue with this patch:
    such regexps are currently rejected as invalid.

The feature seems useful, but how does this affect the numbering of
groups that don't specify a number?  That has to be done right, then
documented in the two manuals.