query-replace-regexp on specific lines

Question

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).

Answer 1

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.

Answer 2

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.