Vim command to delete all but selected lines

Question

I would like to do the inverse operation performed by

:g/pattern/d

i.e Delete all lines in a file which doesn't have pattern

Answer 1

You can use v to select all line without pattern :
:v/pattern/d will achieve what you want to do.

See :help :v

Answer 2

One way to do this is with a shell filter:

:%!grep pattern

This command passes your whole file (%) to a shell (!) which runs it through the grep pattern command and returns the output to your editor window.

Answer 3

You can simply negate the pattern

:g!/pattern/d

This will delete all but the selected lines.