Emacs Lisp: by Example

;;;; Emacs Lisp: by Example
;;; Text Processing and Interaction Facilities
;; February 13 2004
;; Aaron Hawley
;;
;; Emacs Lisp has much of the regular expression power of Perl with
;; the invaluable addition of interactive capabilities.  Below are
;; some examples.  The ability to modify text files is but a taste of
;; the functionality of Emacs and her lisp extension language.  The
;; following functions present some common uses (regular expressions,
;; interaction, highlighting, user prompts) and mistakes (watch for
;; case sensitivity) of writing Emacs Lisp.  The last function proves
;; the point that a search-and-replace (no matter how strong a regular
;; expression) cannot always successfully complete a task that demands
;; human intervention.  These functions were developed in GNU Emacs 21.

(defun mytext-next-all-caps-heading ()
  "Finds a text heading because it is in all caps."
  (interactive)
  (message "Finding text headings ...")
  (push-mark) ;; start from current cursor position
  (let ((case-fold-search nil)) ;; set to case-sensitive.
    (if (re-search-forward
	 "^[^a-z\n]+$"
	 nil ;; not bound
	 t) ;; quell errors
	(message "Found text heading.")
      (message "End of buffer"))))

(defun mytext-unjustify-buffer ()
  "Finds a text heading because it is in all caps."
  (interactive)
  (message "Finding text headings ...")
  (push-mark) ;; start from current cursor position
  (let ((case-fold-search nil)) ;; set to case-sensitive.

    ;; remove any spaces between single consecutive "word
    ;; constituents" (which really just means a single letter).
    (perform-replace
     " \\(\\w\\) +\\(\\w\\)\\b"
     "\\1\\2"
     nil ;; don't query
     t ;; is regexp
     nil)) ;; don't enforce word delimiting
  (message "Line unjustified"))

(defun mytext-indented-lines-to-paragraphs ()
  "Converts single space lines with indentation at beginning of
paragraphs to paragraphs with a blank line between each paragraph."
  (interactive)
  (message "Converting indented lines to paragraphs ...")
  (push-mark) ;; start from current cursor position
  ;; any line starting with blank characters should be changed
  ;; to a double line.
  (perform-replace
   "\n[[:blank:]]+\\b"
   "\n\n"
     nil ;; don't query
     t ;; is regexp
     nil) ;; don't enforce word delimiting
  (message "Indentation converted to paragraphs"))

(defun mytext-previous-word ()
  "Return the word previous to location ``point'' is on--return as a
string."
  (save-excursion (backward-word 1) (current-word)))

;; When using optical character recognition (OCR) scanning technology
;; or simply copying and pasting from print-oriented electronic
;; documents, hyphenation will be present at the end of lines.  To
;; correct this situation with a regular expression is a simple task.
;; Not all hyphens should be removed, however.  Because of the dual
;; meaning of a hyphen, it could very well be necessary for the word's
;; spelling.  So at each hyphenated line, the user is prompted ("yes"
;; or "no") whether to keep or remove the hyphen.
(defun mytext-remove-hyphenated-lines ()
  "Dehyphenates all lines in the buffer, and will wrap the word to the
next line.

Upon the arriving at a hyphenated line, the user will be queried as to
whether the word should maintain the hyphen or be concatenated without
the hyphen."
  (interactive)
  (setq hyphenated-lines 0) ;; counter
  (message "Removing end of line hyphens ...")
  (push-mark) ;; start from current cursor position.
  ;; search repeatedly for a hyphen at the end of a line.
  (while (re-search-forward "-[[:blank:]]*\n\\b"
			    (point-max) ;; go to end of buffer.
			    t) ;; quell errors.
    ;; increment counter.
    (setq hyphenated-lines (1+ hyphenated-lines))
    ;; save-excursion is handy: it allows you to move the cursor
    ;; around, retrieve and return a value but return the cursor back
    ;; to where it started.
    (let* ((hyphenated-word-start (save-excursion
				    (backward-word 1)
				    (point)))
	   (hyphenated-word-end (save-excursion
				  ;; go to end of word.
				  (forward-word 1)
				  (point)))
	   (hyphenated-word (concat
			     (mytext-previous-word)
			     (current-word)))
	   ;; Create an overlay over the span of the hyphenated word.
	   (highlight-overlay (make-overlay hyphenated-word-start
					    hyphenated-word-end)))
      ;; Set font face of the overlay of the high to highlight
      ;; (usually a inverse-video appearance).
      (overlay-put highlight-overlay 'face 'highlight)
      ;; Ask the user if the hyphen is truly part of the word or not.
      ;; FIXME: This could be accomplished much of the time by using
      ;; Emacs's spell-checking facilities.
      (if (y-or-n-p (concat "Remove hyphenation in \""
			    hyphenated-word
			    "\" ?"))
	  ;; If yes, remove hyphen from word.
	  (replace-match "")
	;; Else keep the hyphen in the word.
	(replace-match "-"))
      (delete-overlay highlight-overlay) ;; Remove highlighting.
      (backward-word 1) ;; Go to the beginning of the word.
      (open-line 1) ;; Make end-line, putting (wrapping) word to next
		    ;; line.
      (delete-trailing-whitespace)
      (undo-boundary) ;; have the each iterative modification be a
		      ;; single undo, rather than a single undo of
                      ;; all the work done by this function.
      (forward-line))) ;; Done with this line, continue on next.
  ;; END: report how many lines were fixed.
  (message (concat "Fixed "
		   (number-to-string hyphenated-lines)
		   " hyphenated lines")))