How do I yank all the lines containing a pattern into a buffer?

You can use the “:global” command to yank all the lines containing the pattern into a register and then paste the contents of the register into the buffer:

    :let @a=''

    :g/mypattern/y A

The first command, clears the contents of the register “a”. The second command copies all the lines containing “mypattern” into the register “a”. Note that the capital letter “A” is used to append the matched lines. Now you can paste the contents of register “a” to a buffer using “ap command. If you only want to collect all matches, you can use a different approach. For that run the ‘:s’ command with the flags ‘gn’ so that it won’t actually change the buffer (‘n’ flag) but select each match (‘g’ flag). Combining this with the ‘\=’ part in the replacement part, you can copy each match to e.g. a list. Altogether this looks like this:

    :let list=[]

    :%s/pattern/\=add(list, submatch(0))/gn

Now all matches will be in the list and you can post process them as wanted.