Ex command :v to the rescue!
:v/pattern/d
Using :g! will give you the same result as using :v.
:g finds lines that do match a pattern.
I had a large list of locations. I wanted to keep only the locations containing hyphens and delete the rest so I used this: :v/-/d
:v – find lines not matching your pattern
d – delete matches
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Dan Herbert
Nice!
I always forget about that one. I usually do (for your scenario with hyphens):
:%!grep -v –
n8kowald
Grep with an invert-match, nice!
I'll remember this flag for future greps :)