Use a separate history variable for align-regexp

* lisp/align.el (align-regexp-history): New variable (bug#16891).
(align-regexp): Use it.

etc/NEWS

@@ -31,6 +31,11 @@ otherwise leave it unmarked.
 
 * Changes in Emacs 25.2
 
+---
+** `align-regexp' has a separate history for its interactive argument
+`align-regexp' no longer shares its history with all other
+history-less functions that use `read-string'
+
 +++
 ** The networking code has been reworked so that it's more
 asynchronous than it was (when specifying :nowait t in

lisp/align.el

@@ -802,6 +802,9 @@ See the variable `align-exclude-rules-list' for more details.")
 (defvar align-highlight-overlays nil
   "The current overlays highlighting the text matched by a rule.")
 
+(defvar align-regexp-history nil
+  "Input history for the full user-entered regex in `align-regexp'")
+
 ;; Sample extension rule set, for vhdl-mode.  This should properly be
 ;; in vhdl-mode.el itself.
 
@@ -946,7 +949,7 @@ construct a rule to pass to `align-region', which does the real work."
     (list (region-beginning) (region-end))
     (if current-prefix-arg
 	(list (read-string "Complex align using regexp: "
-			   "\\(\\s-*\\)")
+                          "\\(\\s-*\\)" 'align-regexp-history)
 	      (string-to-number
 	       (read-string
 		"Parenthesis group to modify (justify if negative): " "1"))