How to grep all the Info manuals with a regexp search

Question

People often recommend on emacs.stackexchange.com and stackoverflow.com (in the Emacs tag) that we should learn to "ask Emacs". The built-in Info-mode is not capable of performing a recursive regexp search that can compete with the likes of grep.

Q:  How can I recursively grep the Emacs manual (i.e., all .info files) with a regexp search?

[Keep in mind that some .info files may be gzipped, whereas others will be unzipped. This largely depends upon the install options specified at the time Emacs is built from source.]

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In a related thread on reddit.com, @kaushalmodi made an excellent suggestion to use the ag (aka The Silver Searcher) command-line utility in conjunction with the counsel / ivy / swiper packages, and the following function was provided by said author:

https://www.reddit.com/r/emacs/comments/4vvwr9/grepping_the_info_files_versus_infomode/?ref=share&ref_source=link

(defun counsel-ag-emacs-info (&optional initial-input)
  "Search for a pattern in emacs 'info/' directory using ag.
    INITIAL-INPUT can be given as the initial minibuffer input."
  (interactive)
  (counsel-ag initial-input (car Info-default-directory-list)
              " -z" "Search emacs/elisp info"))

Answer 1

The following solution relies upon three (3) command-line utilities: find; xargs; and zgrep. In putting together the following function, I discovered that grep cannot see inside gizipped files, and not all versions of zgrep are able to search recursively. Inasmuch as zgrep can handle both gzipped and unzipped .info files, the function includes a search for both.

On OSX find/xargs/zgrep come pre-installed and they are located in the /usr/bin directory; and, the car of Info-default-directory-list contains the built-in Emacs manual consisting of several .info files:

With some command-line utilities, Emacs may complain about exiting abnormally with code 1. The zgrep results in this case are nevertheless complete, and running the same commands in the terminal works perfectly without any error messages.

Just type: M-x ask-emacs

(defun ask-emacs ()
"Grep the .info files that are in the CAR of `Info-default-directory-list'."
(interactive)
  (let* (
      (search-term (read-string "Ask Emacs (regex):  "))
      (search-path (directory-file-name (car Info-default-directory-list)))
      (default-directory (file-name-as-directory search-path))
      (initial-grep-command "-inIE --color=always -C2")
      (grep-command (concat
        "find"
        " "
        search-path
        " "
        "\\( -name \\*.gz -o -name \\*.info \\)"
        " "
        "-print0"
        " "
        "|"
        " "
        "xargs"
        " "
        "-0"
        " "
        "zgrep"
        " "
        initial-grep-command
        " "
        search-term
        " "
        search-path)) )
   (compilation-start grep-command 'grep-mode (lambda (mode) "*grep*") nil) ))

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Link to related source with an example to recursively search using zgrep: https://unix.stackexchange.com/a/187753/92940

Answer 2

Another possibility:

In Dired, in a directory in Info-default-directory-list:

1. Use % g (command dired-mark-files-containing-regexp) to mark the Info files containing matches for your regexp.

2. M-: (multi-isearch-files-regexp (dired-get-marked-files)), then type your regexp to search for matches (in the Info files that have matches). (You can turn that into a command if you want.)

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If you use Icicles then instead of step 2 you can do M-: (icicle-search nil nil (format ".*%s.*" REGEXP) t (dired-get-marked-files)), where REGEXP is your regexp. (You can turn that into a command if you want.)

Completion candidates are the lines with REGEXP matches in the Info files. You can type an input pattern to match any of them. Use C-down to cycle among matches, or use C-RET or C-mouse-2 to access any of the matches individually (i.e., in any order).

If REGEXP itself can match more than one line then use it as is, instead of (format ".*%s.*" REGEXP).