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Vim command: g

:g/pattern/cmd
:range g/pattern/cmd

The g command first scans all lines in range and marks those that match pattern.

It then iterates over the marked lines and executes cmd. (See multi-repeat in vim's documentation).

If cmd happens to delete a marked line, the mark disappears.

Because of this two-pass operation, the g command, combined with the move command (m) allow to reverse the order of lines in a buffer:

:g/^/m0

Delete lines that match a pattern

Lines that match a pattern can be easily deleted with the global command combined with d:

:g/pattern/d

The following yanks lines that match a given regular expressions into the register c.

First make sure that the register is cleared:

qcq

Then, the g command is used like so:

:g/pattern/y B

Now, we have the matched lines in the register b.

This works because assigning to an uppercase register appends to the register with its lowercase name.

Here's a file:

some text
# xyz: one
more text
foo bar baz
# xyz: second
final text
# xyz: final words
The end.

I want to extract all text after # xyz:

I can do that with the global command:

:let @a='' | g/^# xyz:/let @A=substitute(getline('.'), '^# xyz: \(.*\)', '\1' . nr2char(10), '')

After executing this command, the register a contains

one
second
final words

:g/^/+d

Note: this command throws error E16: invalid range if the number of lines in the buffer is odd.