How to make sed remove lines not matched by a substitution

Question

I basically want to do this:

cat file | grep \'\' | sed \'s///g\'

without having to

Answer 1

How about:

cat file | sed 'd//'

Will delete matching patterns from the input. Of course, this is opposite of what you want, but maybe you can make an opposite regular expression?

Please not that I'm not completely sure of the syntax, only used it a couple of times some time ago.