Re: string-match bug?

[Barry Margolin]

In article <mailman.12518.1260269458.2239.help-gnu-emacs@gnu.org>,
 Andreas Röhler <andreas.roehler@easy-emacs.de> wrote:

> Barry Margolin wrote:
> > In article <mailman.12469.1260221021.2239.help-gnu-emacs@gnu.org>,
> >  Matthew Dempsky <matthew@dempsky.org> wrote:
> > 
> >> On Mon, Dec 7, 2009 at 12:37 PM, Andreas Röhler
> >> <andreas.roehler@easy-emacs.de> wrote:
> >>> Why should questioned string respond here it contains an empty string at 
> >>> position 0?
> >>> Makes no sense for me.
> >> Here's an analogy: (string-match "xyzzy" "fooxyzzybar") returns 3.
> >> This is because the first 5 characters starting at position 3 are
> >> "xyzzy", the same as the first string parameter.  The significance of
> >> 5 here is the length of "xyzzy".
> >>
> >> Similarly, (string-match "" "foo") returns 0.  This is because the
> >> first 0 characters starting at position are "", the same as the first
> >> string parameter.
> > 
> > Here's another example of a limit case:
> > 
> > (string-match "a*" "b") returns 0, because a* matches zero or more a's, 
> > and there are zero a's at position 0.
> > 
> 
> Hmm, interesting
> 
> IMHO that differs:
> 
> (string-match "a*" "b") asks for a non-occurrence too. So "0" of first 
> position is plausible.

What's the difference between a non-occurrence and a zero-length 
occurrence?

The nice thing about regular expressions is that the following syllogism 
holds:

If 
(string-match (concat regex1 regex2) (concat str1 str2))
then
(and (string-match regex1 str1) (string-match regex2 str2))

You need the empty regex case to work for the limiting case of this.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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