Remove the optional KEEP-ORDER argument to regexp-opt

This argument was added for the 'or' clause in rx, but it turned out
to be a bad idea (bug#37659), and there seems to be little other use
for it.

* lisp/emacs-lisp/regexp-opt.el (regexp-opt): Remove KEEP-ORDER.
* doc/lispref/searching.texi (Regexp Functions):
* etc/NEWS: Remove it from the documentation.
* test/lisp/emacs-lisp/regexp-opt-tests.el (regexp-opt-test--match-all)
(regexp-opt-test--check-perm, regexp-opt-test--explain-perm)
(regexp-opt-keep-order, regexp-opt-longest-match): Simplify test.

4 files changed, 19 insertions, 84 deletions

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index 5f4509a8b43..1f6db0643e8 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -1745,7 +1745,7 @@ any special characters.
 @end defun
 
 @cindex optimize regexp
-@defun regexp-opt strings &optional paren keep-order
+@defun regexp-opt strings &optional paren
 This function returns an efficient regular expression that will match
 any of the strings in the list @var{strings}.  This is useful when you
 need to make matching or searching as fast as possible---for example,
@@ -1783,11 +1783,8 @@ if it is necessary to ensure that a postfix operator appended to
 it will apply to the whole expression.
 @end table
 
-The optional argument @var{keep-order}, if non-@code{nil}, forces the
-match to be performed in the order given, as if the strings were made
-into a regexp by joining them with the @samp{\|} operator.  If nil or
-omitted, the returned regexp will always match the longest string
-possible.
+The returned regexp is ordered in such a way that it will always match
+the longest string possible.
 
 Up to reordering, the resulting regexp of @code{regexp-opt} is
 equivalent to but usually more efficient than that of a simplified
diff --git a/etc/NEWS b/etc/NEWS
index 312869fe57e..380ac71260d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -3527,13 +3527,6 @@ This is currently supported on GNUish hosts and on modern versions of
 MS-Windows.
 
 +++
-** The function 'regexp-opt' accepts an additional optional argument.
-By default, the regexp returned by 'regexp-opt' may match the strings
-in any order.  If the new third argument is non-nil, the match is
-guaranteed to be performed in the order given, as if the strings were
-made into a regexp by joining them with '\|'.
-
-+++
 ** The function 'regexp-opt', when given an empty list of strings, now
 returns a regexp that never matches anything, which is an identity for
 this operation.  Previously, the empty string was returned in this
diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index 2cce4e63539..35a5fda184f 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -84,7 +84,7 @@
 ;;; Code:
 
 ;;;###autoload
-(defun regexp-opt (strings &optional paren keep-order)
+(defun regexp-opt (strings &optional paren)
   "Return a regexp to match a string in the list STRINGS.
 Each member of STRINGS is treated as a fixed string, not as a regexp.
 Optional PAREN specifies how the returned regexp is surrounded by
@@ -114,11 +114,8 @@ nil
     necessary to ensure that a postfix operator appended to it will
     apply to the whole expression.
 
-The optional argument KEEP-ORDER, if non-nil, forces the match to
-be performed in the order given, as if the strings were made into
-a regexp by joining them with the `\\|' operator.  If nil or
-omitted, the returned regexp is will always match the longest
-string possible.
+The returned regexp is ordered in such a way that it will always
+match the longest string possible.
 
 Up to reordering, the resulting regexp is equivalent to but
 usually more efficient than that of a simplified version:
@@ -140,34 +137,12 @@ usually more efficient than that of a simplified version:
            (completion-ignore-case nil)
            (completion-regexp-list nil)
            (open (cond ((stringp paren) paren) (paren "\\(")))
-           (re
-            (cond
-             ;; No strings: return an unmatchable regexp.
-             ((null strings)
-              (concat (or open "\\(?:") regexp-unmatchable "\\)"))
-
-             ;; The algorithm will generate a pattern that matches
-             ;; longer strings in the list before shorter.  If the
-             ;; list order matters, then no string must come after a
-             ;; proper prefix of that string.  To check this, verify
-             ;; that a straight or-pattern matches each string
-             ;; entirely.
-             ((and keep-order
-                   (let* ((case-fold-search nil)
-                          (alts (mapconcat #'regexp-quote strings "\\|")))
-                     (and (let ((s strings))
-                            (while (and s
-                                        (string-match alts (car s))
-                                        (= (match-end 0) (length (car s))))
-                              (setq s (cdr s)))
-                            ;; If we exited early, we found evidence that
-                            ;; regexp-opt-group cannot be used.
-                            s)
-                          (concat (or open "\\(?:") alts "\\)")))))
-             (t
-              (regexp-opt-group
-               (delete-dups (sort (copy-sequence strings) 'string-lessp))
-               (or open t) (not open))))))
+           (re (if strings
+                   (regexp-opt-group
+                    (delete-dups (sort (copy-sequence strings) 'string-lessp))
+                    (or open t) (not open))
+                 ;; No strings: return an unmatchable regexp.
+                 (concat (or open "\\(?:") regexp-unmatchable "\\)"))))
       (cond ((eq paren 'words)
              (concat "\\<" re "\\>"))
             ((eq paren 'symbols)
diff --git a/test/lisp/emacs-lisp/regexp-opt-tests.el b/test/lisp/emacs-lisp/regexp-opt-tests.el
index 9b4567c72cc..0179ac4f1f4 100644
--- a/test/lisp/emacs-lisp/regexp-opt-tests.el
+++ b/test/lisp/emacs-lisp/regexp-opt-tests.el
@@ -47,43 +47,13 @@
   (mapcar (lambda (i) (regexp-opt-test--permutation i list))
           (number-sequence 0 (1- (regexp-opt-test--factorial (length list))))))
 
-(defun regexp-opt-test--match-all (words re)
-  (mapcar (lambda (w) (and (string-match re w)
-                           (match-string 0 w)))
-          words))
-
-(defun regexp-opt-test--check-perm (perm)
-  (let* ((ref-re (mapconcat #'regexp-quote perm "\\|"))
-         (opt-re (regexp-opt perm nil t))
-         (ref (regexp-opt-test--match-all perm ref-re))
-         (opt (regexp-opt-test--match-all perm opt-re)))
-    (equal opt ref)))
-
-(defun regexp-opt-test--explain-perm (perm)
-  (let* ((ref-re (mapconcat #'regexp-quote perm "\\|"))
-         (opt-re (regexp-opt perm nil t))
-         (ref (regexp-opt-test--match-all perm ref-re))
-         (opt (regexp-opt-test--match-all perm opt-re)))
-    (concat "\n"
-            (format "Naïve regexp:     %s\n" ref-re)
-            (format "Optimized regexp: %s\n" opt-re)
-            (format "Got:      %s\n" opt)
-            (format "Expected: %s\n" ref))))
-
-(put 'regexp-opt-test--check-perm 'ert-explainer 'regexp-opt-test--explain-perm)
-
-(ert-deftest regexp-opt-keep-order ()
-  "Check that KEEP-ORDER works."
-  (dolist (perm (regexp-opt-test--permutations '("abc" "bca" "cab")))
-    (should (regexp-opt-test--check-perm perm)))
-  (dolist (perm (regexp-opt-test--permutations '("abc" "ab" "bca" "bc")))
-    (should (regexp-opt-test--check-perm perm)))
-  (dolist (perm (regexp-opt-test--permutations '("abxy" "cdxy")))
-    (should (regexp-opt-test--check-perm perm)))
-  (dolist (perm (regexp-opt-test--permutations '("afgx" "bfgx" "afgy" "bfgy")))
-    (should (regexp-opt-test--check-perm perm)))
-  (dolist (perm (regexp-opt-test--permutations '("a" "ab" "ac" "abc")))
-    (should (regexp-opt-test--check-perm perm))))
+(ert-deftest regexp-opt-longest-match ()
+  "Check that the regexp always matches as much as possible."
+  (let ((s "abcd"))
+    (dolist (perm (regexp-opt-test--permutations '("a" "ab" "ac" "abc")))
+      (should (equal (and (string-match (regexp-opt perm) s)
+                          (match-string 0 s))
+                     "abc")))))
 
 (ert-deftest regexp-opt-charset ()
   (should (equal (regexp-opt-charset '(?a ?b ?a)) "[ab]"))