Fix regexp-opt documentation (bug #17862)

* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
* doc/lispref/searching.texi (Regexp Functions): Update PAREN doc.

1 files changed, 36 insertions, 10 deletions

diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index b1e132a76e3..27db4773b1d 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -86,18 +86,44 @@
 ;;;###autoload
 (defun regexp-opt (strings &optional paren)
   "Return a regexp to match a string in the list STRINGS.
-Each string should be unique in STRINGS and should not contain any regexps,
-quoted or not.  If optional PAREN is non-nil, ensure that the returned regexp
-is enclosed by at least one regexp grouping construct.
-The returned regexp is typically more efficient than the equivalent regexp:
+Each string should be unique in STRINGS and should not contain
+any regexps, quoted or not.  Optional PAREN specifies how the
+returned regexp is surrounded by grouping constructs.
 
- (let ((open (if PAREN \"\\\\(\" \"\")) (close (if PAREN \"\\\\)\" \"\")))
-   (concat open (mapconcat \\='regexp-quote STRINGS \"\\\\|\") close))
+The optional argument PAREN can be any of the following:
 
-If PAREN is `words', then the resulting regexp is additionally surrounded
-by \\=\\< and \\>.
-If PAREN is `symbols', then the resulting regexp is additionally surrounded
-by \\=\\_< and \\_>."
+a string
+    the resulting regexp is preceded by PAREN and followed by
+    \\), e.g.  use \"\\\\(?1:\" to produce an explicitly numbered
+    group.
+
+`words'
+    the resulting regexp is surrounded by \\=\\<\\( and \\)\\>.
+
+`symbols'
+    the resulting regexp is surrounded by \\_<\\( and \\)\\_>.
+
+non-nil
+    the resulting regexp is surrounded by \\( and \\).
+
+nil
+    the resulting regexp is surrounded by \\(?: and \\), if it is
+    necessary to ensure that a postfix operator appended to it will
+    apply to the whole expression.
+
+The resulting regexp is equivalent to but usually more efficient
+than that of a simplified version:
+
+ (defun simplified-regexp-opt (strings &optional paren)
+   (let ((parens
+          (cond ((stringp paren)       (cons paren \"\\\\)\"))
+                ((eq paren 'words)    '(\"\\\\\\=<\\\\(\" . \"\\\\)\\\\>\"))
+                ((eq paren 'symbols) '(\"\\\\_<\\\\(\" . \"\\\\)\\\\_>\"))
+                ((null paren)          '(\"\\\\(?:\" . \"\\\\)\"))
+                (t                       '(\"\\\\(\" . \"\\\\)\")))))
+     (concat (car paren)
+             (mapconcat 'regexp-quote strings \"\\\\|\")
+             (cdr paren))))"
   (save-match-data
     ;; Recurse on the sorted list.
     (let* ((max-lisp-eval-depth 10000)