bug#20307: [PATCH] make regexp-opt return a no-match return value with empty input

["Mattias Engdegård" <mattiase@acm.org>]

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>> Sorry, I don't understand. Do we in general distinguish nil from the empty list in documentation?
> 
> I don't know.  I thought the reader might not be aware of their
> equivalence, so being explicit would be better.
> 
>> Or did you mean that the phrase should be "If STRINGS is the empty list..."?
> 
> Yes, that would take care of the issue.

Now done in the doc string and in searching.texi.

New patch attached.


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From 944a6100d5d2563fdd355f9f91a6190117a79efc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Mon, 25 Feb 2019 15:22:02 +0100
Subject: [PATCH] Correct regexp-opt return value for empty string list

When regexp-opt is called with an empty list of strings, return a regexp
that doesn't match anything instead of the empty string (Bug#20307).

* doc/lispref/searching.texi (Regular Expression Functions):
* etc/NEWS:
Document the new behaviour.
* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
Return a never-match regexp for empty inputs.
---
 doc/lispref/searching.texi    |  3 +++
 etc/NEWS                      |  6 ++++++
 lisp/emacs-lisp/regexp-opt.el | 23 +++++++++++++++--------
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index fb7f48474d..38e6204055 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -960,6 +960,9 @@ possible.  A hand-tuned regular expression can sometimes be slightly
 more efficient, but is almost never worth the effort.}.
 @c E.g., see https://debbugs.gnu.org/2816
 
+If @var{strings} is the empty list, the return value is a regexp that
+never matches anything.
+
 The optional argument @var{paren} can be any of the following:
 
 @table @asis
diff --git a/etc/NEWS b/etc/NEWS
index 7c95988ff5..65eb9ba1af 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1649,6 +1649,12 @@ 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
+case.
+
 \f
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 
diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index d0c5f2d3fc..4404b905a6 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -90,6 +90,9 @@ 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.
 
+If STRINGS is the empty list, the return value is a regexp that
+never matches anything.
+
 The optional argument PAREN can be any of the following:
 
 a string
@@ -140,14 +143,18 @@ usually more efficient than that of a simplified version:
 	   (sorted-strings (delete-dups
 			    (sort (copy-sequence strings) 'string-lessp)))
 	   (re
-            ;; If NOREORDER is non-nil and the list contains a prefix
-            ;; of another string, we give up all attempts at optimisation.
-            ;; There is plenty of room for improvement (Bug#34641).
-            (if (and noreorder (regexp-opt--contains-prefix sorted-strings))
-                (concat (or open "\\(?:")
-                        (mapconcat #'regexp-quote strings "\\|")
-                        "\\)")
-              (regexp-opt-group sorted-strings (or open t) (not open)))))
+            (cond
+             ;; No strings: return a\` which cannot match anything.
+             ((null strings)
+              (concat (or open "\\(?:") "a\\`\\)"))
+             ;; If we cannot reorder, give up all attempts at
+             ;; optimisation.  There is room for improvement (Bug#34641).
+             ((and noreorder (regexp-opt--contains-prefix sorted-strings))
+              (concat (or open "\\(?:")
+                      (mapconcat #'regexp-quote strings "\\|")
+                      "\\)"))
+             (t
+              (regexp-opt-group sorted-strings (or open t) (not open))))))
       (cond ((eq paren 'words)
 	     (concat "\\<" re "\\>"))
 	    ((eq paren 'symbols)
-- 
2.17.2 (Apple Git-113)