;;; regexp-opt.el --- generate efficient regexps to match strings -*- lexical-binding: t -*-
;; Copyright (C) 1994-2024 Free Software Foundation, Inc.
;; Author: Simon Marshall <simon@gnu.org>
;; Maintainer: emacs-devel@gnu.org
;; Keywords: strings, regexps, extensions
;; This file is part of GNU Emacs.
;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
;;; Commentary:
;; The "opt" in "regexp-opt" stands for "optim\\(al\\|i[sz]e\\)".
;;
;; This package generates a regexp from a given list of strings (which matches
;; one of those strings) so that the regexp generated by:
;;
;; (regexp-opt strings)
;;
;; is equivalent to, but more efficient than, the regexp generated by:
;;
;; (mapconcat 'regexp-quote strings "\\|")
;;
;; For example:
;;
;; (let ((strings '("cond" "if" "when" "unless" "while"
;; "let" "let*" "progn" "prog1" "prog2"
;; "save-restriction" "save-excursion" "save-window-excursion"
;; "save-current-buffer" "save-match-data"
;; "catch" "throw" "unwind-protect" "condition-case")))
;; (concat "(" (regexp-opt strings t) "\\>"))
;; => "(\\(c\\(atch\\|ond\\(ition-case\\)?\\)\\|if\\|let\\*?\\|prog[12n]\\|save-\\(current-buffer\\|excursion\\|match-data\\|restriction\\|window-excursion\\)\\|throw\\|un\\(less\\|wind-protect\\)\\|wh\\(en\\|ile\\)\\)\\>"
;;
;; Searching using the above example `regexp-opt' regexp takes approximately
;; two-thirds of the time taken using the equivalent `mapconcat' regexp.
;; Since this package was written to produce efficient regexps, not regexps
;; efficiently, it is probably not a good idea to in-line too many calls in
;; your code, unless you use the following trick with `eval-when-compile':
;;
;; (defvar definition-regexp
;; (eval-when-compile
;; (concat "^("
;; (regexp-opt '("defun" "defsubst" "defmacro" "defalias"
;; "defvar" "defconst") t)
;; "\\>")))
;;
;; The `byte-compile' code will be as if you had defined the variable thus:
;;
;; (defvar definition-regexp
;; "^(\\(def\\(alias\\|const\\|macro\\|subst\\|un\\|var\\)\\)\\>")
;;
;; Note that if you use this trick for all instances of `regexp-opt' and
;; `regexp-opt-depth' in your code, regexp-opt.el would only have to be loaded
;; at compile time. But note also that using this trick means that should
;; regexp-opt.el be changed, perhaps to fix a bug or to add a feature to
;; improve the efficiency of `regexp-opt' regexps, you would have to recompile
;; your code for such changes to have effect in your code.
;; Originally written for font-lock.el, from an idea from Stig's hl319.el, with
;; thanks for ideas also to Michael Ernst, Bob Glickstein, Dan Nicolaescu and
;; Stefan Monnier.
;; No doubt `regexp-opt' doesn't always produce optimal regexps, so code, ideas
;; or any other information to improve things are welcome.
;;
;; One possible improvement would be to compile '("aa" "ab" "ba" "bb")
;; into "[ab][ab]" rather than "a[ab]\\|b[ab]". I'm not sure it's worth
;; it but if someone knows how to do it without going through too many
;; contortions, I'm all ears.
;;; Code:
;;;###autoload
(defun regexp-opt (strings &optional paren)
"Return a regexp to match a string in the list STRINGS.
Each member of STRINGS is treated as a fixed string, not as a regexp.
Optional PAREN specifies how the returned regexp is surrounded by
grouping constructs.
If STRINGS is the empty list, the return value is a regexp that
never matches anything.
The optional argument PAREN can be any of the following:
a string
the resulting regexp is preceded by PAREN and followed by
\\), e.g. use \"\\\\(?1:\" to produce an explicitly numbered
group.
`words'
the resulting regexp is surrounded by \\=\\<\\( and \\)\\>.
`symbols'
the resulting regexp is surrounded by \\_<\\( and \\)\\_>.
non-nil
the resulting regexp is surrounded by \\( and \\).
nil
the resulting regexp is surrounded by \\(?: and \\), if it is
necessary to ensure that a postfix operator appended to it will
apply to the whole expression.
The returned regexp is ordered in such a way that it will always
match the longest string possible.
Up to reordering, the resulting regexp is equivalent to but
usually more efficient than that of a simplified version:
(defun simplified-regexp-opt (strings &optional paren)
(let ((parens
(cond ((stringp paren) (cons paren \"\\\\)\"))
((eq paren \\='words) \\='(\"\\\\\\=<\\\\(\" . \"\\\\)\\\\>\"))
((eq paren \\='symbols) \\='(\"\\\\_<\\\\(\" . \"\\\\)\\\\_>\"))
((null paren) \\='(\"\\\\(?:\" . \"\\\\)\"))
(t \\='(\"\\\\(\" . \"\\\\)\")))))
(concat (car parens)
(mapconcat \\='regexp-quote strings \"\\\\|\")
(cdr parens))))"
(save-match-data
;; Recurse on the sorted list.
(let* ((max-lisp-eval-depth 10000)
(completion-ignore-case nil)
(completion-regexp-list nil)
(open (cond ((stringp paren) paren) (paren "\\(")))
(re (if strings
(regexp-opt-group
(delete-dups (sort (copy-sequence strings) 'string-lessp))
(or open t) (not open))
;; No strings: return an unmatchable regexp.
(concat (or open "\\(?:") regexp-unmatchable "\\)"))))
(cond ((eq paren 'words)
(concat "\\<" re "\\>"))
((eq paren 'symbols)
(concat "\\_<" re "\\_>"))
(t re)))))
;;;###autoload
(defun regexp-opt-depth (regexp)
"Return the depth of REGEXP.
This means the number of non-shy regexp grouping constructs
\(parenthesized expressions) in REGEXP."
(save-match-data
;; Hack to signal an error if REGEXP does not have balanced parentheses.
(string-match regexp "")
;; Count the number of open parentheses in REGEXP.
(let ((count 0) start last)
(while (string-match "\\\\(\\(\\?[0-9]*:\\)?" regexp start)
(setq start (match-end 0)) ; Start of next search.
(when (and (not (match-beginning 1))
(subregexp-context-p regexp (match-beginning 0) last))
;; It's not a shy group and it's not inside brackets or after
;; a backslash: it's really a group-open marker.
(setq last start) ; Speed up next regexp-opt-re-context-p.
(setq count (1+ count))))
count)))
;;; Workhorse functions.
(defun regexp-opt-group (strings &optional paren lax)
"Return a regexp to match a string in the sorted list STRINGS.
If PAREN non-nil, output regexp parentheses around returned regexp.
If LAX non-nil, don't output parentheses if it doesn't require them.
Merges keywords to avoid backtracking in Emacs's regexp matcher."
;; The basic idea is to find the shortest common prefix or suffix, remove it
;; and recurse. If there is no prefix, we divide the list into two so that
;; (at least) one half will have at least a one-character common prefix.
;; Also we delay the addition of grouping parenthesis as long as possible
;; until we're sure we need them, and try to remove one-character sequences
;; so we can use character sets rather than grouping parenthesis.
(let* ((open-group (cond ((stringp paren) paren) (paren "\\(?:") (t "")))
(close-group (if paren "\\)" ""))
(open-charset (if lax "" open-group))
(close-charset (if lax "" close-group)))
(cond
;;
;; If there are no strings, just return the empty string.
((= (length strings) 0)
"")
;;
;; If there is only one string, just return it.
((= (length strings) 1)
(if (= (length (car strings)) 1)
(concat open-charset (regexp-quote (car strings)) close-charset)
(concat open-group (regexp-quote (car strings)) close-group)))
;;
;; If there is an empty string, remove it and recurse on the rest.
((= (length (car strings)) 0)
(concat open-charset
(regexp-opt-group (cdr strings) t t) "?"
close-charset))
;;
;; If there are several one-char strings, use charsets
((and (= (length (car strings)) 1)
(let ((strs (cdr strings)))
(while (and strs (/= (length (car strs)) 1))
(pop strs))
strs))
(let (letters rest)
;; Collect one-char strings
(dolist (s strings)
(if (= (length s) 1) (push (string-to-char s) letters) (push s rest)))
(if rest
;; several one-char strings: take them and recurse
;; on the rest (first so as to match the longest).
(concat open-group
(regexp-opt-group (nreverse rest))
"\\|" (regexp-opt-charset letters)
close-group)
;; all are one-char strings: just return a character set.
(concat open-charset
(regexp-opt-charset letters)
close-charset))))
;;
;; We have a list of different length strings.
(t
(let ((prefix (try-completion "" strings)))
(if (> (length prefix) 0)
;; common prefix: take it and recurse on the suffixes.
(let* ((n (length prefix))
(suffixes (mapcar (lambda (s) (substring s n)) strings)))
(concat open-group
(regexp-quote prefix)
(regexp-opt-group suffixes t t)
close-group))
(let* ((sgnirts (mapcar #'reverse strings))
(xiffus (try-completion "" sgnirts)))
(if (> (length xiffus) 0)
;; common suffix: take it and recurse on the prefixes.
(let* ((n (- (length xiffus)))
(prefixes
;; Sorting is necessary in cases such as ("ad" "d").
(sort (mapcar (lambda (s) (substring s 0 n)) strings)
'string-lessp)))
(concat open-group
(regexp-opt-group prefixes t t)
(regexp-quote (nreverse xiffus))
close-group))
;; Otherwise, divide the list into those that start with a
;; particular letter and those that do not, and recurse on them.
(let* ((char (substring-no-properties (car strings) 0 1))
(half1 (all-completions char strings))
(half2 (nthcdr (length half1) strings)))
(concat open-group
(regexp-opt-group half1)
"\\|" (regexp-opt-group half2)
close-group))))))))))
(defun regexp-opt-charset (chars)
"Return a regexp to match a character in CHARS.
CHARS should be a list of characters.
If CHARS is the empty list, the return value is a regexp that
never matches anything."
;; The basic idea is to find character ranges. Also we take care in the
;; position of character set meta characters in the character set regexp.
;;
(let* ((charmap (make-char-table 'regexp-opt-charset))
(start -1) (end -2)
(charset "")
(bracket "") (dash "") (caret ""))
;;
;; Make a character map but extract character set meta characters.
(dolist (char chars)
(cond
((eq char ?\])
(setq bracket "]"))
((eq char ?^)
(setq caret "^"))
((eq char ?-)
(setq dash "-"))
(t
(aset charmap char t))))
;;
;; Make a character set from the map using ranges where applicable.
(map-char-table
(lambda (c v)
(when v
(if (consp c)
(if (= (1- (car c)) end) (setq end (cdr c))
(if (> end (+ start 2))
(setq charset (format "%s%c-%c" charset start end))
(while (>= end start)
(setq charset (format "%s%c" charset start))
(setq start (1+ start))))
(setq start (car c) end (cdr c)))
(if (= (1- c) end) (setq end c)
(if (> end (+ start 2))
(setq charset (format "%s%c-%c" charset start end))
(while (>= end start)
(setq charset (format "%s%c" charset start))
(setq start (1+ start))))
(setq start c end c)))))
charmap)
(when (>= end start)
(if (> end (+ start 2))
(setq charset (format "%s%c-%c" charset start end))
(while (>= end start)
(setq charset (format "%s%c" charset start))
(setq start (1+ start)))))
;; Make sure that ] is first, ^ is not first, - is first or last.
(let ((all (concat bracket charset caret dash)))
(pcase (length all)
(0 regexp-unmatchable)
(1 (regexp-quote all))
(_ (if (string-equal all "^-")
"[-^]"
(concat "[" all "]")))))))
(provide 'regexp-opt)
;;; regexp-opt.el ends here