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After running (grep) the grep buffer appears with matches. Some of the matches are not relevant, and it would be good to remove them manually. This way I can use the grep buffer to hold only those matches which are relevant to the current investigation.
Is this possible?
Absolutely. And it is extremely useful to do that. M-x flush-lines and M-x keep-lines are your friends, as well as just C-k etc.
(The first thing you need to do is of course C-x C-q, to make the buffer writable.)
If you use library Grep+ (grep+.el) then you have these commands, which remove comment lines and toggle such removal:
grepp-remove-comments, grepp-toggle-comments, which are bound in grep buffers to ; and M-;, respectively.
(Grep+ also gives you commands that make it easy to have multiple grep buffers: + (grepp-new-buffer), b (grepp-choose-buffer), and r (grepp-rename-buffer-to-last).)
There are also other buffers that are read-only by default and which it can sometimes be useful to use C-x C-q and then edit them, in particular (IMHO), *Messages*, *Help*, and *Colors* (from M-x list-colors-display). Info used to even have a command that lets you edit the current node.
Unfortunately (IMHO), Emacs has generally moved away from such use, making it harder. For example, *Help* and *Messages* buffers now defined keys in their respective modes, which means that to edit them normally you need to either (1) change to another mode or (2) use C-q to be able to insert certain characters.
If you are an evil user, you can have a binding like zww 'read-only-mode, that toggle read-only-map. Disable read-only-mode and then use evil-delete line (i.e. dd). Of course this does not automatically remove all the lines like flush-lines, however, if you want remove a whole region without using complex regular expressions, then this is your friend.
winnow.el provides a handy little minor mode designed for this very purpose, intended for use in compilation-mode buffers (or derivatives, which includes grep-mode).
It enables key bindings equivalent to flush-lines and keep-lines, and takes care of the read-only-ness of the text for you.
Key Function
x winnow-exclude-lines
m winnow-match-lines
