bug#17862: 24.3; regexp-opt docstring is incorrect

[immerrr again <immerrr@gmail.com>]

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On Tue, Jul 1, 2014 at 7:22 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>> I suppose you could use it to add an explicit group number.
>> Is it worth it?
>
> I don't think so.
>

Removing this parameter or not is probably a subject of a different
discussion concerning backward compatibility and whatnot. Within the
scope of this bug, I'd like to suggest the following patch to fix the
issue.

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From ba401fc2d5ee090ed065e9f0c0d90ef8a3b1fea4 Mon Sep 17 00:00:00 2001
From: immerrr <immerrr@gmail.com>
Date: Sun, 7 Feb 2016 12:46:37 +0300
Subject: [PATCH] Fix regexp-opt documentation (bug #17862)

* lisp/emacs-lisp/regexp-opt.el (regexp-opt): update PAREN doc

* doc/lispref/searching.texi (Regexp Functions): update PAREN doc
---
 doc/lispref/searching.texi    | 33 ++++++++++++++++++++++-----------
 lisp/emacs-lisp/regexp-opt.el | 29 +++++++++++++++++++++--------
 2 files changed, 43 insertions(+), 19 deletions(-)

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index 1243d72..e6de768 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -946,23 +946,34 @@ Regexp Functions
 more efficient, but is almost never worth the effort.}.
 @c E.g., see http://debbugs.gnu.org/2816
 
-If the optional argument @var{paren} is non-@code{nil}, then the
-returned regular expression is always enclosed by at least one
-parentheses-grouping construct.  If @var{paren} is @code{words}, then
-that construct is additionally surrounded by @samp{\<} and @samp{\>};
-alternatively, if @var{paren} is @code{symbols}, then that construct
-is additionally surrounded by @samp{\_<} and @samp{\_>}
-(@code{symbols} is often appropriate when matching
-programming-language keywords and the like).
+The optional argument @var{paren} can be any of the following:
+
+a string
+    the resulting regexp is surrounded by @code{paren} and @samp{\)},
+    e.g.  use @samp{"\\(?1:"} to produce an explicitly numbered group.
+
+@code{words}
+    the resulting regexp is surrounded by @samp{\<\(} and @samp{\)\>}.
+
+@code{symbols}
+    the resulting regexp is surrounded by @samp{\_<\(} and @samp{\)\_>}
+    (this is often appropriate when maching programming-language
+    keywords and the like).
+
+non-@code{nil}
+    the resulting regexp is surrounded by @samp{\(} and @samp{\)}.
+
+Otherwise the resulting regexp is surrounded by @samp{\(?:} and
+@samp{\)}.
 
 This simplified definition of @code{regexp-opt} produces a
 regular expression which is equivalent to the actual value
-(but not as efficient):
+(but typically more efficient):
 
 @example
 (defun regexp-opt (strings &optional paren)
-  (let ((open-paren (if paren "\\(" ""))
-        (close-paren (if paren "\\)" "")))
+  (let ((open-paren (make-open-paren paren))
+        (close-paren (make-close-paren paren)))
     (concat open-paren
             (mapconcat 'regexp-quote strings "\\|")
             close-paren)))
diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index b1e132a..6a3912c 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -86,18 +86,31 @@
 ;;;###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.
+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.
+
 The returned regexp is typically more efficient than the equivalent regexp:
 
- (let ((open (if PAREN \"\\\\(\" \"\")) (close (if PAREN \"\\\\)\" \"\")))
+ (let ((open (make-open-paren PAREN)) (close (make-close-paren PAREN)))
    (concat open (mapconcat \\='regexp-quote STRINGS \"\\\\|\") close))
 
-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 \\_>."
+PAREN can be any of the following:
+
+a string
+    the resulting regexp is surrounded by PAREN and \\), 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 \\).
+
+Otherwise the resulting regexp is surrounded by \\(?: and \\)."
   (save-match-data
     ;; Recurse on the sorted list.
     (let* ((max-lisp-eval-depth 10000)
-- 
2.7.0