How to delete lines which do not contains a particular string?
I would like to delete within vim all lines which would not match a regular expression
pattern. Something like :g/!one +string/d (but off course, bang does not work here;
I do not know the syntax and this is why I am posting).
Specifically, I want to delete all lines which do not have an "=" sign. But I would like
to have a more generic receipe which would involve regex patterns.