[cl-ppcre-devel] searching backward?

[Edi Weitz]

On Fri, 16 Feb 2007 12:58:42 +0100, Edi Weitz <edi at agharta.de> wrote:

> Of course, if you're really adventurous, you could look at the
> source code of CREATE-SCANNER-AUX in CL-PPCRE and think about
> efficient variants of ADVANCE-FN for searching backwards.  My guess
> (from looking at the Emacs C code for two minutes) is that this is
> more or less what Emacs is doing as well.

I forgot: In an empty Emacs *scratch* buffer type "aaaaaaaa" (eight
#\a's) and put point in the middle (after the fourth #\a).  Then
evaluate (using eval-expression) the following

  (re-search-forward "a+")

This should give you 9 and is what one would expect - the regex engine
matches the four #\a's after point.

Now put point back in the middle of the string and evaluate

  (re-search-backward "a+")

That'll give you 4, i.e. the engine matches (only) the fourth #\a - a
string of length one.

I think this confirms my point that Emacs somehow has to go backwards
and step by step while the regular expressions themselves still "match
forwards" - so to say.  It also shows that scanning backwards somehow
destroys the semantics of some of the regex constituent - "*" or "+"
used to mean "longest possible match", but is a string of length one
really the longest match?