Highlight regexp sub-expressions

* doc/emacs/search.texi (Search Customizations): Document it.

* lisp/isearch.el (search-highlight-submatches): New variable.
(isearch-group-1, isearch-group-2, isearch-group-3)
(isearch-group-4, isearch-group-5): New faces.
(isearch-highlight): Use them.
(isearch-dehighlight): Ditto (bug#6227).

doc/emacs/search.texi

@@ -1977,6 +1977,16 @@ performs case folding and lax-whitespace matching.
 using the @code{isearch} face.  This highlighting can be disabled by
 setting the variable @code{search-highlight} to @code{nil}.
 
+@vindex search-highlight-submatches
+  When searching for regular expressions (with @kbd{C-u C-s}, for
+instance), subexpressions receive special highlighting depending on
+the @code{search-highlight-submatches} variable.  If this variable is
+zero, no special highlighting is done, but if this is larger than
+zero, subexpressions will be matched with
+@code{isearch-group-}@samp{X} faces.  For instance, when searching for
+@samp{foo-\([0-9]+\)}, the part matched by @samp{[0-9]+} will be
+highlighted with the @code{isearch-group-1} face.
+
 @cindex lazy highlighting customizations
 @vindex isearch-lazy-highlight
 @cindex @code{lazy-highlight} face

etc/NEWS

@@ -1034,6 +1034,12 @@ window after starting).  This variable defaults to nil.
 
 ** Miscellaneous
 
++++
+*** Interactive regular expression search now uses faces for sub-groups.
+'C-u C-s foo-\([0-9]+\)' will now use the 'isearch-group-1' face on the
+part of the regexp that matches the sub-expression "[0-9]+".  This is
+controlled by the 'search-highlight-submatches' variable.
+
 ---
 *** New user option 'reveal-auto-hide'.
 If non-nil (the default), revealed text is automatically hidden when

lisp/isearch.el

@@ -269,6 +269,19 @@ are `word-search-regexp' \(`\\[isearch-toggle-word]'), `isearch-symbol-regexp'
   "Non-nil means incremental search highlights the current match."
   :type 'boolean)
 
+(defcustom search-highlight-submatches 5
+  "Highlight regexp subexpressions of the current regexp match.
+An integer means highlight regexp subexpressions up to the
+specified maximal number.
+
+When 0, do not highlight regexp subexpressions.
+
+The faces used to do the highlights are named `isearch-group-1'
+and so on, and if you increase this variable from the default,
+you have to add more of these faces."
+  :type 'integer
+  :version "28.1")
+
 (defface isearch
   '((((class color) (min-colors 88) (background light))
      ;; The background must not be too dark, for that means
@@ -3654,6 +3667,57 @@ since they have special meaning in a regexp."
 ;; Highlighting
 
 (defvar isearch-overlay nil)
+(defvar isearch-submatches-overlays nil)
+
+(defface isearch-group-1
+  '((((class color) (background light))
+     (:background "#ff00ff" :foreground "lightskyblue1"))
+    (((class color) (background dark))
+     (:background "palevioletred3" :foreground "brown4"))
+    (t (:inverse-video t)))
+  "Face for highlighting Isearch sub-group matches (first sub-group)."
+  :group 'isearch
+  :version "28.1")
+
+(defface isearch-group-2
+  '((((class color) (background light))
+     (:background "#d000d0" :foreground "lightskyblue1"))
+    (((class color) (background dark))
+     (:background "#be698f" :foreground "black"))
+    (t (:inverse-video t)))
+  "Face for highlighting Isearch sub-group matches (second sub-group)."
+  :group 'isearch
+  :version "28.1")
+
+(defface isearch-group-3
+  '((((class color) (background light))
+     (:background "#a000a0" :foreground "lightskyblue1"))
+    (((class color) (background dark))
+     (:background "#a06080" :foreground "brown4"))
+    (t (:inverse-video t)))
+  "Face for highlighting Isearch sub-group matches (third sub-group)."
+  :group 'isearch
+  :version "28.1")
+
+(defface isearch-group-4
+  '((((class color) (background light))
+     (:background "#800080" :foreground "lightskyblue1"))
+    (((class color) (background dark))
+     (:background "#905070" :foreground "brown4"))
+    (t (:inverse-video t)))
+  "Face for highlighting Isearch sub-group matches (fourth sub-group)."
+  :group 'isearch
+  :version "28.1")
+
+(defface isearch-group-5
+  '((((class color) (background light))
+     (:background "#600060" :foreground "lightskyblue1"))
+    (((class color) (background dark))
+     (:background "#804060" :foreground "black"))
+    (t (:inverse-video t)))
+  "Face for highlighting Isearch sub-group matches (fifth sub-group)."
+  :group 'isearch
+  :version "28.1")
 
 (defun isearch-highlight (beg end)
   (if search-highlight
@@ -3664,11 +3728,28 @@ since they have special meaning in a regexp."
 	(setq isearch-overlay (make-overlay beg end))
 	;; 1001 is higher than lazy's 1000 and ediff's 100+
 	(overlay-put isearch-overlay 'priority 1001)
-	(overlay-put isearch-overlay 'face isearch-face))))
+	(overlay-put isearch-overlay 'face isearch-face)))
+  (when (and (integerp search-highlight-submatches)
+	     (> search-highlight-submatches 0)
+	     isearch-regexp)
+    (mapc 'delete-overlay isearch-submatches-overlays)
+    (setq isearch-submatches-overlays nil)
+    (let ((i 0) ov)
+      (while (<= i search-highlight-submatches)
+	(when (match-beginning i)
+	  (setq ov (make-overlay (match-beginning i) (match-end i)))
+	  (overlay-put ov 'face (intern-soft (format "isearch-group-%d" i)))
+	  (overlay-put ov 'priority 1002)
+	  (push ov isearch-submatches-overlays))
+	(setq i (1+ i))))))
 
 (defun isearch-dehighlight ()
   (when isearch-overlay
-    (delete-overlay isearch-overlay)))
+    (delete-overlay isearch-overlay))
+  (when search-highlight-submatches
+    (mapc 'delete-overlay isearch-submatches-overlays)
+    (setq isearch-submatches-overlays nil)))
+
 
 ;; isearch-lazy-highlight feature
 ;; by Bob Glickstein <http://www.zanshin.com/~bobg/>