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

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> 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`.  ]

> 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?


        Stefan


(pcase-defmacro re-match (re)
  "Matches a string if that string matches RE.
RE should be a regular expression (a string).
It can use the special syntax \\(?VAR: to bind a sub-match
to variable VAR.  All other subgroups are treated as shy.

Multiple uses of this macro in a single `pcase' are not optimized
together, so don't expect lex-like performance.  But in order for
such optimization to be possible in some distant future, back-references
are not supported."
  (let ((start 0)
        (last 0)
        (new-re '())
        (vars '())
        (gn 0))
    (while (string-match "\\\\(\\(?:\\?\\([-[:alnum:]]*\\):\\)?" re start)
      (setq start (match-end 0))
      (let ((beg (match-beginning 0))
            (name (match-string 1 re)))
        ;; Skip false positives, either backslash-escaped or within [...].
        (when (subregexp-context-p re start last)
          (cond
           ((null name)
            (push (concat (substring re last beg) "\\(?:") new-re))
           ((string-match "\\`[0-9]" name)
            (error "Variable can't start with a digit: %S" name))
           (t
            (let* ((var (intern name))
                   (id (cdr (assq var vars))))
              (unless id
                (setq gn (1+ gn))
                (setq id gn)
                (push (cons var gn) vars))
              (push (concat (substring re last beg) (format "\\(?%d:" id))
                    new-re))))
          (setq last start))))
    (push (substring re last) new-re)
    (setq new-re (mapconcat #'identity (nreverse new-re) ""))
    `(and (pred stringp)
          (app (lambda (s)
                 (save-match-data
                   (when (string-match ,new-re s)
                     (vector ,@(mapcar (lambda (x) `(match-string ,(cdr x) s))
                                       vars)))))
               (,'\` [,@(mapcar (lambda (x) (list '\, (car x))) vars)])))))