Add 'rx' pattern for pcase.

* lisp/emacs-lisp/rx.el (rx): New pcase macro.
* test/lisp/emacs-lisp/rx-tests.el (rx-pcase): Add unit test.

lisp/emacs-lisp/rx.el

@@ -1169,6 +1169,62 @@ enclosed in `(and ...)'.
 	 (rx-to-string `(and ,@regexps) t))
 	(t
 	 (rx-to-string (car regexps) t))))
+
+
+(pcase-defmacro rx (&rest regexps)
+  "Build a `pcase' pattern matching `rx' regexps.
+The REGEXPS are interpreted as by `rx'.  The pattern matches if
+the regular expression so constructed matches the object, as if
+by `string-match'.
+
+In addition to the usual `rx' constructs, REGEXPS can contain the
+following constructs:
+
+  (let VAR FORM...)  creates a new explicitly numbered submatch
+                     that matches FORM and binds the match to
+                     VAR.
+  (backref VAR)      creates a backreference to the submatch
+                     introduced by a previous (let VAR ...)
+                     construct.
+
+The VARs are associated with explicitly numbered submatches
+starting from 1.  Multiple occurrences of the same VAR refer to
+the same submatch.
+
+If a case matches, the match data is modified as usual so you can
+use it in the case body, but you still have to pass the correct
+string as argument to `match-string'."
+  (let* ((vars ())
+         (rx-constituents
+          `((let
+             ,(lambda (form)
+                (rx-check form)
+                (let ((var (cadr form)))
+                  (cl-check-type var symbol)
+                  (let ((i (or (cl-position var vars :test #'eq)
+                               (prog1 (length vars)
+                                 (setq vars `(,@vars ,var))))))
+                    (rx-form `(submatch-n ,(1+ i) ,@(cddr form))))))
+             1 nil)
+            (backref
+             ,(lambda (form)
+                (rx-check form)
+                (rx-backref
+                 `(backref ,(let ((var (cadr form)))
+                              (if (integerp var) var
+                                (1+ (cl-position var vars :test #'eq)))))))
+             1 1
+             ,(lambda (var)
+                (cond ((integerp var) (rx-check-backref var))
+                      ((memq var vars) t)
+                      (t (error "rx `backref' variable must be one of %s: %s"
+                                vars var)))))
+            ,@rx-constituents))
+         (regexp (rx-to-string `(seq ,@regexps) :no-group)))
+    `(and (pred (string-match ,regexp))
+          ,@(cl-loop for i from 1
+                     for var in vars
+                     collect `(app (match-string ,i) ,var)))))
 
 ;; ;; sregex.el replacement