Add standard unmatchable regexp

Add `regexp-unmatchable' as a standard unmatchable regexp, defined as
"\\`a\\`".  Use it where such a regexp is needed, replacing slower
expressions in several places.
From a suggestion by Philippe Schnoebelen.

* lisp/subr.el (regexp-unmatchable): New defconst.
* etc/NEWS (Lisp Changes): Mention `regexp-unmatchable'.
* doc/lispref/searching.texi (Regexp Functions): Document it.
* lisp/emacs-lisp/regexp-opt.el (regexp-opt)
* lisp/progmodes/cc-defs.el (cc-conditional-require-after-load)
(c-make-keywords-re)
* lisp/progmodes/cc-engine.el (c-beginning-of-statement-1)
(c-forward-<>-arglist-recur, c-forward-decl-or-cast-1)
(c-looking-at-decl-block)
* lisp/progmodes/cc-fonts.el (c-doc-line-join-re)
(c-doc-bright-comment-start-re)
* lisp/progmodes/cc-langs.el (c-populate-syntax-table)
(c-assignment-op-regexp)
(c-block-comment-ender-regexp, c-font-lock-comment-end-skip)
(c-block-comment-start-regexp, c-line-comment-start-regexp)
(c-doc-comment-start-regexp, c-decl-start-colon-kwd-re)
(c-type-decl-prefix-key, c-type-decl-operator-prefix-key)
(c-pre-id-bracelist-key, c-enum-clause-introduction-re)
(c-nonlabel-token-2-key)
* lisp/progmodes/cc-mode.el (c-doc-fl-decl-start, c-doc-fl-decl-end)
* lisp/progmodes/cc-vars.el (c-noise-macro-with-parens-name-re)
(c-noise-macro-name-re, c-make-noise-macro-regexps)
* lisp/progmodes/octave.el (octave-help-mode)
* lisp/vc/vc-bzr.el (vc-bzr-log-view-mode, vc-bzr-revision-completion-table)
* lisp/vc/vc-git.el (vc-git-log-view-mode)
* lisp/vc/vc-hg.el (vc-hg-log-view-mode)
* lisp/vc/vc-mtn.el (vc-mtn-log-view-mode):
Use `regexp-unmatchable'.
* lisp/textmodes/ispell.el (ispell-non-empty-string):
Use `regexp-unmatchable', fixing a broken never-match regexp.

1 files changed, 6 insertions, 4 deletions

diff --git a/lisp/progmodes/cc-engine.el b/lisp/progmodes/cc-engine.el
index ed8310d0e67..41bab270daa 100644
--- a/lisp/progmodes/cc-engine.el
+++ b/lisp/progmodes/cc-engine.el
@@ -907,7 +907,7 @@ comment at the start of cc-engine.el for more info."
 	stack
 	;; Regexp which matches "for", "if", etc.
 	(cond-key (or c-opt-block-stmt-key
-		      "a\\`"))	; Doesn't match anything.
+		      regexp-unmatchable))
 	;; Return value.
 	(ret 'same)
 	;; Positions of the last three sexps or bounds we've stopped at.
@@ -7646,7 +7646,9 @@ comment at the start of cc-engine.el for more info."
 		(progn
 		  (c-forward-syntactic-ws)
 		  (when (or (and c-record-type-identifiers all-types)
-			    (not (equal c-inside-<>-type-key "\\(a\\`\\)")))
+			    (not (equal c-inside-<>-type-key
+					(concat
+					 "\\(" regexp-unmatchable "\\)"))))
 		    (c-forward-syntactic-ws)
 		    (cond
 		     ((eq (char-after) ??)
@@ -9253,7 +9255,7 @@ This function might do hidden buffer changes."
       ;; Skip over type decl prefix operators.  (Note similar code in
       ;; `c-forward-declarator'.)
       (if (and c-recognize-typeless-decls
-	       (equal c-type-decl-prefix-key "a\\`")) ; Regexp which doesn't match
+	       (equal c-type-decl-prefix-key regexp-unmatchable))
 	  (when (eq (char-after) ?\()
 	    (progn
 	      (setq paren-depth (1+ paren-depth))
@@ -10886,7 +10888,7 @@ comment at the start of cc-engine.el for more info."
 	      ;; legal because it's part of a "compound keyword" like
 	      ;; "enum class".	Of course, if c-after-brace-list-key
 	      ;; is nil, we can skip the test.
-	      (or (equal c-after-brace-list-key "a\\`") ; Regexp which doesn't match
+	      (or (equal c-after-brace-list-key regexp-unmatchable)
 		  (save-match-data
 		    (save-excursion
 		      (not