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

[Juri Linkov <juri@jurta.org>]

> 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)