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, 2 insertions, 2 deletions

diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
index bd62fc754ab..e4ff9f019d3 100644
--- a/lisp/progmodes/cc-mode.el
+++ b/lisp/progmodes/cc-mode.el
@@ -1825,7 +1825,7 @@ Note that this is a strict tail, so won't match, e.g. \"0x....\".")
   ;; by `c-doc-line-join-re'), return the position of the first line of the
   ;; sequence.  Otherwise, return nil.  Point has no significance at entry to
   ;; and exit from this function.
-  (when (not (equal c-doc-line-join-re "a\\`"))
+  (when (not (equal c-doc-line-join-re regexp-unmatchable))
     (goto-char pos)
     (back-to-indentation)
     (and (or (looking-at c-comment-start-regexp)
@@ -1842,7 +1842,7 @@ Note that this is a strict tail, so won't match, e.g. \"0x....\".")
   ;; marker (as defined by `c-doc-line-join-re), return the position of
   ;; the BOL at the end of the sequence.  Otherwise, return nil.  Point has no
   ;; significance at entry to and exit from this function.
-  (when (not (equal c-doc-line-join-re "a\\`"))
+  (when (not (equal c-doc-line-join-re regexp-unmatchable))
     (goto-char pos)
     (back-to-indentation)
     (let ((here (point)))