@c Internal functions: regexp-opt-group
+@defvar regexp-unmatchable
+This variable contains a regexp that is guaranteed not to match any
+string at all. It is particularly useful as default value for
+variables that may be set to a pattern that actually matches
+something.
+@end defvar
+
@node Regexp Search
@section Regular Expression Searching
@cindex regular expression searching
this operation. Previously, the empty string was returned in this
case.
+** New constant 'regexp-unmatchable' contains a never-matching regexp.
+It is a convenient and readable way to specify a regexp that should
+not match anything, and is as fast as any such regexp can be.
+
\f
* Changes in Emacs 27.1 on Non-Free Operating Systems
(sort (copy-sequence strings) 'string-lessp)))
(re
(cond
- ;; No strings: return a\` which cannot match anything.
+ ;; No strings: return an unmatchable regexp.
((null strings)
- (concat (or open "\\(?:") "a\\`\\)"))
+ (concat (or open "\\(?:") regexp-unmatchable "\\)"))
;; If we cannot reorder, give up all attempts at
;; optimisation. There is room for improvement (Bug#34641).
((and keep-order (regexp-opt--contains-prefix sorted-strings))
(make-local-variable 'comint-password-prompt-regexp)
;; This is a regexp that can't match anything.
;; ange-ftp has its own ways of handling passwords.
- (setq comint-password-prompt-regexp "\\`a\\`")
+ (setq comint-password-prompt-regexp regexp-unmatchable)
(make-local-variable 'paragraph-start)
(setq paragraph-start comint-prompt-regexp))
(progn
(require 'font-lock)
(let (font-lock-keywords)
- (font-lock-compile-keywords '("a\\`")) ; doesn't match anything.
+ (font-lock-compile-keywords (list regexp-unmatchable))
font-lock-keywords))))
\f
;; Produce a regexp that doesn't match anything.
(if adorn
- "\\(a\\`\\)"
- "a\\`")))
+ (concat "\\(" regexp-unmatchable "\\)")
+ regexp-unmatchable)))
(put 'c-make-keywords-re 'lisp-indent-function 1)
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.
(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) ??)
;; 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))
;; 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
\f
;;; Doc comments.
-(defvar c-doc-line-join-re "a\\`")
+(defvar c-doc-line-join-re regexp-unmatchable)
;; Matches a join of two lines in a doc comment.
;; This should not be changed directly, but instead set by
;; `c-setup-doc-comment-style'. This variable is used in `c-find-decl-spots'
;; in (e.g.) autodoc style comments to bridge the gap between a "@\n" at an
;; EOL and the token following "//!" on the next line.
-(defvar c-doc-bright-comment-start-re "a\\`")
+(defvar c-doc-bright-comment-start-re regexp-unmatchable)
;; Matches the start of a "bright" comment, one whose contents may be
;; fontified by, e.g., `c-font-lock-declarations'.
(c-make-keywords-re 'appendable
(c-lang-const c-cpp-include-directives))
"[ \t]*")
- "a\\`")) ; Doesn't match anything
+ regexp-unmatchable))
(c-lang-defvar c-cpp-include-key (c-lang-const c-cpp-include-key))
(c-lang-defconst c-opt-cpp-macro-define
(c--set-difference (c-lang-const c-assignment-operators)
'("=")
:test 'string-equal)))
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-defvar c-assignment-op-regexp
(c-lang-const c-assignment-op-regexp))
;; language)
t (if (c-lang-const c-block-comment-ender)
(regexp-quote (c-lang-const c-block-comment-ender))
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-defvar c-block-comment-ender-regexp
(c-lang-const c-block-comment-ender-regexp))
;; `font-lock-comment-delimiter-face'.
t (if (c-lang-const c-block-comment-ender)
(concat "[ \t]*" (c-lang-const c-block-comment-ender-regexp))
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-setvar font-lock-comment-end-skip
(c-lang-const c-font-lock-comment-end-skip))
;; language)
t (if (c-lang-const c-block-comment-starter)
(regexp-quote (c-lang-const c-block-comment-starter))
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-defvar c-block-comment-start-regexp
(c-lang-const c-block-comment-start-regexp))
;; language; it does in all 7 CC Mode languages).
t (if (c-lang-const c-line-comment-starter)
(regexp-quote (c-lang-const c-line-comment-starter))
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-defvar c-line-comment-start-regexp
(c-lang-const c-line-comment-start-regexp))
(c-lang-defconst c-doc-comment-start-regexp
"Regexp to match the start of documentation comments."
- t "a\\`" ; Doesn't match anything.
+ t regexp-unmatchable
;; From font-lock.el: `doxygen' uses /*! while others use /**.
(c c++ objc) "/\\*[*!]"
java "/\\*\\*"
"Regexp matching a keyword that is followed by a colon, where
the whole construct can precede a declaration.
E.g. \"public:\" in C++."
- t "a\\`" ; Doesn't match anything.
+ t regexp-unmatchable
c++ (c-make-keywords-re t (c-lang-const c-protection-kwds)))
(c-lang-defvar c-decl-start-colon-kwd-re
(c-lang-const c-decl-start-colon-kwd-re))
t (if (c-lang-const c-type-modifier-kwds)
(concat (regexp-opt (c-lang-const c-type-modifier-kwds) t) "\\>")
;; Default to a regexp that never matches.
- "a\\`")
+ regexp-unmatchable)
;; Check that there's no "=" afterwards to avoid matching tokens
;; like "*=".
(c objc) (concat "\\("
as the end of the operator. Identifier syntax is in effect when
this is matched \(see `c-identifier-syntax-table')."
t ;; Default to a regexp that never matches.
- "a\\`"
+ regexp-unmatchable
;; Check that there's no "=" afterwards to avoid matching tokens
;; like "*=".
(c objc) (concat "\\(\\*\\)"
(c-lang-defconst c-pre-id-bracelist-key
"A regexp matching tokens which, preceding an identifier, signify a bracelist.
"
- t "a\\`" ; Doesn't match anything.
+ t regexp-unmatchable
c++ "new\\([^[:alnum:]_$]\\|$\\)\\|&&?\\(\\S.\\|$\\)")
(c-lang-defvar c-pre-id-bracelist-key (c-lang-const c-pre-id-bracelist-key))
;; before the '{' of the enum list, to avoid searching too far.
"[^][{};/#=]*"
"{")
- "a\\`")) ; Doesn't match anything.
+ regexp-unmatchable))
(c-lang-defvar c-enum-clause-introduction-re
(c-lang-const c-enum-clause-introduction-re))
"Regexp matching things that can't occur two symbols before a colon in
a label construct. This catches C++'s inheritance construct \"class foo
: bar\". Only used if `c-recognize-colon-labels' is set."
- t "a\\`" ; Doesn't match anything.
+ t regexp-unmatchable
c++ (c-make-keywords-re t '("class")))
(c-lang-defvar c-nonlabel-token-2-key (c-lang-const c-nonlabel-token-2-key))
;; 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)
;; 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)))
:group 'c)
;; Initialize the next two to a regexp which never matches.
-(defvar c-noise-macro-with-parens-name-re "a\\`")
+(defvar c-noise-macro-with-parens-name-re regexp-unmatchable)
(make-variable-buffer-local 'c-noise-macro-with-parens-name-re)
-(defvar c-noise-macro-name-re "a\\`")
+(defvar c-noise-macro-name-re regexp-unmatchable)
(make-variable-buffer-local 'c-noise-macro-name-re)
(defcustom c-noise-macro-names nil
;; Convert `c-noise-macro-names' and `c-noise-macro-with-parens-names' into
;; `c-noise-macro-name-re' and `c-noise-macro-with-parens-name-re'.
(setq c-noise-macro-with-parens-name-re
- (cond ((null c-noise-macro-with-parens-names) "a\\`") ; Never matches.
+ (cond ((null c-noise-macro-with-parens-names) regexp-unmatchable)
((consp c-noise-macro-with-parens-names)
(concat (regexp-opt c-noise-macro-with-parens-names t)
"\\([^[:alnum:]_$]\\|$\\)"))
(t (error "c-make-noise-macro-regexps: \
c-noise-macro-with-parens-names is invalid: %s" c-noise-macro-with-parens-names))))
(setq c-noise-macro-name-re
- (cond ((null c-noise-macro-names) "a\\`") ; Never matches anything.
+ (cond ((null c-noise-macro-names) regexp-unmatchable)
((consp c-noise-macro-names)
(concat (regexp-opt c-noise-macro-names t)
"\\([^[:alnum:]_$]\\|$\\)"))
grep-mode-line-matches)
;; compilation-directory-matcher can't be nil, so we set it to a regexp that
;; can never match.
- (set (make-local-variable 'compilation-directory-matcher) '("\\`a\\`"))
+ (set (make-local-variable 'compilation-directory-matcher)
+ (list regexp-unmatchable))
(set (make-local-variable 'compilation-process-setup-function)
'grep-process-setup)
(set (make-local-variable 'compilation-disable-input) t)
(eval-and-compile (require 'help-mode))
;; Don't highlight `EXAMPLE' as elisp symbols by using a regexp that
;; can never match.
- (setq-local help-xref-symbol-regexp "x\\`"))
+ (setq-local help-xref-symbol-regexp regexp-unmatchable))
(defun octave-help (fn)
"Display the documentation of FN."
;; for discoverability:
(defalias 'flatten-list 'flatten-tree)
+;; The initial anchoring is for better performance in searching matches.
+(defconst regexp-unmatchable "\\`a\\`"
+ "Standard regexp guaranteed not to match any string at all.")
+
;;; subr.el ends here
(defun ispell-non-empty-string (string)
(if (or (not string) (string-equal string ""))
- "\\'\\`" ; An unmatchable string if string is null.
+ regexp-unmatchable
(regexp-quote string)))
(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\\]\\)?"
((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))
(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))
(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)
(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.
projects / emacs.git / commitdiff