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.

4 files changed, 6 insertions, 5 deletions

diff --git a/lisp/vc/vc-bzr.el b/lisp/vc/vc-bzr.el
index ab5a449cd3d..ee1646cae5a 100644
--- a/lisp/vc/vc-bzr.el
+++ b/lisp/vc/vc-bzr.el
@@ -702,7 +702,7 @@ or a superior directory.")
   (remove-hook 'log-view-mode-hook 'vc-bzr-log-view-mode) ;Deactivate the hack.
   (require 'add-log)
   (set (make-local-variable 'log-view-per-file-logs) nil)
-  (set (make-local-variable 'log-view-file-re) "\\`a\\`")
+  (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-message-re)
        (if (eq vc-log-view-type 'short)
 	   "^ *\\([0-9.]+\\): \\(.*?\\)[ \t]+\\([0-9]\\{4\\}-[0-9]\\{2\\}-[0-9]\\{2\\}\\)\\( \\[merge\\]\\)?"
@@ -1319,7 +1319,8 @@ stream.  Standard error output is discarded."
      ((string-match "\\`annotate:" string)
       (completion-table-with-context
        (substring string 0 (match-end 0))
-       (apply-partially #'completion-table-with-terminator '(":" . "\\`a\\`")
+       (apply-partially #'completion-table-with-terminator
+                        (cons ":" regexp-unmatchable)
                         #'completion-file-name-table)
        (substring string (match-end 0)) pred action))
 
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 192e6cf68f6..61c13026cc5 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1087,7 +1087,7 @@ If LIMIT is a revision string, use it as an end-revision."
 (define-derived-mode vc-git-log-view-mode log-view-mode "Git-Log-View"
   (require 'add-log) ;; We need the faces add-log.
   ;; Don't have file markers, so use impossible regexp.
-  (set (make-local-variable 'log-view-file-re) "\\`a\\`")
+  (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-per-file-logs) nil)
   (set (make-local-variable 'log-view-message-re)
        (if (not (eq vc-log-view-type 'long))
diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index d3f132dae70..876d824ceac 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -444,7 +444,7 @@ If LIMIT is non-nil, show no more than this many entries."
 
 (define-derived-mode vc-hg-log-view-mode log-view-mode "Hg-Log-View"
   (require 'add-log) ;; we need the add-log faces
-  (set (make-local-variable 'log-view-file-re) "\\`a\\`")
+  (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-per-file-logs) nil)
   (set (make-local-variable 'log-view-message-re)
        (if (eq vc-log-view-type 'short)
diff --git a/lisp/vc/vc-mtn.el b/lisp/vc/vc-mtn.el
index f0b12489c1b..91cc28021cf 100644
--- a/lisp/vc/vc-mtn.el
+++ b/lisp/vc/vc-mtn.el
@@ -240,7 +240,7 @@ If LIMIT is non-nil, show no more than this many entries."
 
 (define-derived-mode vc-mtn-log-view-mode log-view-mode "Mtn-Log-View"
   ;; Don't match anything.
-  (set (make-local-variable 'log-view-file-re) "\\`a\\`")
+  (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-per-file-logs) nil)
   ;; TODO: Use a more precise regexp than "[ |/]+" to avoid false positives
   ;; in the ChangeLog text.