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how does one do a query-replace-regexp to add a prefix string but not on EVERY line but rather just lines with text (or start with specific text?)

i know how to add a prefix with query-replace-regexp but that adds the prefix to every line (even empty ones).

  • Have you tried to use ^ to match the beginning of the line? – DoMiNeLa10 Nov 8 '16 at 16:18
  • Is this query-replace, query-replaceregexp, or replace-regexp`? – icarus Nov 8 '16 at 16:26
  • hi, its query-replace-regexp. i can replace using ^ but that covers ALL lines and i dont want to add a prefix in non text (empty) lines – zeltak Nov 8 '16 at 16:32
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Another way to approach this is to use occur to select the lines you want to modify, and then apply your modification in the occur buffer:

  1. M-x occur, enter the regexp that matches the lines you want to edit
  2. Switch to the occur window
  3. Type e to enter occur-edit-mode
  4. M-C-% to call query-replace-regexp, do your replacements
  5. C-c C-c to leave occur-edit-mode
  6. Move back to your original window, where the modifications you made in step 4 will be made.

This does the same thing that @icarus suggests, but uses two different regexps: one to select the line, and a second to modify the selected lines. This approach requires a few more steps, but might be easier if @icarus solution requires a very complicated regexp.

You could also do variants of this using helm-occur or other packages, but what I describe here works in regular Emacs without any additional packages.

  • great answer. I was going for simple, as the OP said he wanted to modify the prefix, so he needed to specify the regular expression anyhow, no matter how complicated. – icarus Nov 8 '16 at 22:38
  • Yes, I think your answer is the correct one for this instance. My answer is going to be more useful when the regexps get hairy. You can also use multiple-cursors in the occur buffer, which opens up different possibilities. – Tyler Nov 8 '16 at 23:46
  • hmmm so i tried this but i cant get the changes made in step 4 to reflect in the original buffer. for example a word "page" which i want to replace with "start". i search with occur, it opens a buffer, i change in the occur buffer (after pressing e) the words page to start, press C-C but nothing happens in the original buffer. any clue? what am i missing? – zeltak Nov 10 '16 at 6:42
  • It should happen automatically, if you've entered occur-edit-mode (by pressing e). What version of Emacs are you using? This works fine for me on Emacs 25.1.50.2 – Tyler Nov 10 '16 at 17:00
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If you want to add a prefix of hello to all lines which start with a letter, you can use C-M-% ^\([a-zA-Z]\) hello \1

The idea is to use \(...\) to capture stuff and \1 to represent the first captured group. Here I am using [a-zA-Z] to represent a single letter using the normal unix conventions. It would be better if I said [[:alpha:]] instead.

If you want to add the prefix to non-empty lines then the critical observation you need is that a line is non-empty if it has a character on it! So you want to replace ^\(.\) with prefix \1.

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