bug#12321: 24.2.50; `read-regexp' parameter PROMPT

[Juri Linkov]

> First, there is the business of automatically handling `: ' for the
> PROMPT.  I thought we had finally gotten away from this kind of thing.
> It just gives callers less control.

There are three contradicting requirements:

1. Maintain backward-compatibility for the PROMPT arg
   in all existing calls to `read-regexp'.

2. Do not append automatically ": " to the end of PROMPT.
   As bug#7567 demonstrates, more control for callers is necessary
   to be able to use `read-regexp' in such specific contexts
   as in `query-replace-read-from' that constructs prompt
   according to the special format "%s (default %s -> %s): ".

3. The code should not be so "clever" in supporting
   two these options above at the same time, as you said in
   http://thread.gmane.org/gmane.emacs.devel/134852/focus=134854

But from these three options I see no better thing to do than
to allow `read-regexp' to be "clever" (like `read-buffer'):

=== modified file 'lisp/replace.el'
--- lisp/replace.el     2012-09-09 22:15:24 +0000
+++ lisp/replace.el     2012-09-20 08:15:20 +0000
@@ -597,10 +597,12 @@ (defun read-regexp (prompt &optional def
         (history-add-new-input nil)
         (input
          (read-from-minibuffer
-          (if default-value
-              (format "%s (default %s): " prompt
-                      (query-replace-descr default-value))
-            (format "%s: " prompt))
+          (if (string-match-p ":[ \t]*\\'" prompt)
+              prompt
+            (if default-value
+                (format "%s (default %s): " prompt
+                        (query-replace-descr default-value))
+              (format "%s: " prompt)))
           nil nil nil 'regexp-history defaults t)))
     (if (equal input "")
        (or default-value input)