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From: Luc Teirlinck <teirllm@dms.auburn.edu>
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Subject: `match-data' set improperly
Date: Wed, 12 Feb 2003 18:54:33 -0600 (CST)
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Matthew Swift wrote:

    The documentation (TeXinfo and docstrings) is pretty clear that
    (match-string N) when submatch N>0 does not match anything is nil,
    and = it is also clear that (match-string 0) is like the case for
    N>0 but referring = to the entire match instead of a submatch.
    There is no suggestion that match-data is ever undefined, once one
    match has been done,

The following in the Elisp manual (Chapter: Searching and Matching, Simple
Match Data Access) suggests exactly that to me:

       A search which fails may or may not alter the match data.  In the
    past, a failing search did not do this, but we may change it in the
    future.

Sincerely,

Luc.