Sometimes I want to delete lines that doesn’t match a particular pattern, for those you can do a regex

/^\(.*WORD\)\@!.*$

Breaking it down

^ means start of the line \(.*WORD\) it’s the atom word been searched \@! it’s the important command that negates the atom word (it’s not exactly a negation, please do a :help @! for more info). .*$ everything else until the end of line.

Now I could do this to delete them:

:%g/^\(.*WORD\)\@!.*$/norm!dd

There are other very interesting use cases for this, for example, you could match words that are not followed by some other pattern:

/foo\(bar\)\@!

This will find all cases of foo not followed by bar.