Edit multiple lines at once in Emacs

It's as simple as this: C-x r t

Some examples are here: http://ergoemacs.org/emacs/emacs_string-rectangle_ascii-art.html

One of the solutions is using CUA mode. Activate cua mode with M-x cua-mode, select rectangle begin: first press C-Enter then move cursor with standard movement commands to make selection, now pressing enter at any time will cycle cursor through corners of the rectangle enabling you to prepend or append text to the selection.

You can use the following commands (and keys) to accomplish this:

• open-rectangle (C-x, r, o) add spaces
• kill-rectangle (C-x, r, k) delete
• clear-rectangle (C-x, r, c) replace with spaces
• M-x string-insert-rectangle fill with specified text

Here is a complete description of those features: http://www.gnu.org/software/emacs/manual/html_node/emacs/Rectangles.html

Rectangles are for simple stuff like deleting the same amount of spaces in adjacent lines.

Otherwise keyboard macros are the way to go.

I believe you are looking for the cua-mode that was suggested by boskom. http://www.vimeo.com/1168225?pg=embed&sec=1168225 this screencast might give you an idea of how to use this.

The answers show above are for inserting text in columns. TextMate's "Edit Each Line in Selection" inserts the same text in each line regardless of the length of each lines. I'm learning Lisp now, so as an exercise I wrote a function to do this:

(defun append-to-lines (text-to-be-inserted)
  ;;Appends text to each line in region
  (interactive "sEnter text to append: ")
  (save-excursion
    (let (point-ln mark-ln initial-ln final-ln count)
      (barf-if-buffer-read-only)
      (setq point-ln (line-number-at-pos))
      (exchange-point-and-mark)
      (setq mark-ln (line-number-at-pos))
      (if (< point-ln mark-ln)
          (progn (setq initial-ln point-ln final-ln mark-ln)
                 (exchange-point-and-mark))
        (setq initial-ln mark-ln final-ln point-ln))
      (setq count initial-ln)
      (while (<= count final-ln)
        (progn (move-end-of-line 1)
               (insert text-to-be-inserted)
               (next-line)
               (setq count (1+ count))))
      (message "From line %d to line %d." initial-ln final-ln ))))

You first make a selection that includes all the lines you want to affect and then run the function with M-x append-to-lines.

Step by step, how to change prefix for multiple lines with rectangle:

Press CTRL-x, then SPACE. This starts "rectangle mode".
Now move cursor around to select the area you want - like first column of multiple lines.

Press ALT-x and type string-rectangle.
This starts "string rectangle 'action'" where you type what to do with each line.
(That's the C-x r t in @allyourcode 's answer).
So type -. And hit enter.

You'll get a "dash" on each line.

There are other operations you can do with the selection, like delete etc. - see the link in @allyourcode 's answer.

I didn't understand this from others here, nor from docs, sorry :) I hope someone will find it useful.