Re: rx vs sregex - a "match" to the death

[Martin Stone Davis]

Oliver Scholz wrote:

> Martin Stone Davis <m0davis@pacbell.net> writes:
> [...]
>
> > While I was comparing the two functions, I could not find if/how rx
> > handles numbered backreferences.  It's done in sregex like this:
> >
> > ;;   (sregexq (group (or "Go" "Run"))
> > ;;            ", Spot, "
> > ;;            (backref 1))             =>  "\\(Go\\|Run\\), Spot, \\1"
> >
> > How do you do the same using rx?
>
> [...]
>
> Well, you could use (regexp "\\1"), but this is of course not
> satisfying. I am not aware of any other way to use backrefs with `rx'.
>
> However, I was not aware that you actually can use backrefs within a
> regexp. I tried 
>
> (progn (looking-at "\\w+")
>        (forward-char 1)
>        (re-search-forward "\\0" nil t))
>
> But that did not lead to the result that you would expect if backrefs
> are/were supported within regexps.
>
> Could you provide an example where this is meaningful?

I think you're using backreference wrongly.  Try the one I gave.It 
matches "Go, Spot, Go" OR "Run, Spot, Run".   If I run

(re-search-forward (sregexq (group (or "Go" "Run"))
                            ", Spot, "
                            (backref 1))

It finds either of those two phrases.

-Martin