bug#27659: 26.0.50; Add string-matched-text: string-match + match-string

[Philipp Stephani <p.stephani2@gmail.com>]

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Stefan Monnier <monnier@iro.umontreal.ca> schrieb am Fr., 21. Juli 2017 um
15:34 Uhr:

> >> This looks useful, but I think it would be even better to add it
> >> as a pcase macro to be composable (see attached patch).
>
> Hey, very nice.  Please add it to rx.el.
> [ But please change `backref-var` to just `backref` (you can distinguish
>   the two based on the type of the argument, I think).  I guess one
>   could also argue that you could similarly rename the `let` to
>   `group-n`.  ]
>


Pushed as ad4eff3b905dbc32e2d38bfec1e4f93eceec288d. I've renamed
backref-var to backref as you suggested, but left `let' because I think
that feature is important enough to deserve a short, common name.


>
> > Although, i must admit i am not fluent in `rx' syntaxis, i find your idea
> > very nice.
>
> If you prefer the standard/cryptic regexp syntax, I posted a similar
> thingy in the past (see below).
>
> This lets you do
>
>     (pcase "foo-123"
>       ((re-match "[[:alpha:]]+-\\(?num:[0-9]+\\)")
>        num))
>     => "123"
>
> Maybe I should install it in pcase.el?
>
>
>
Sure! I'd suggest to change the syntax to be compatible with other
languages:

\(?<abc>[0-9]+\) or \(?'abc'[0-9+\) (Perl and .NET)
\(?P<abc>[0-9]+\) (Python)

These languages also have syntax for named backreferences, though that's
less important.

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