I know that Google lets you search for an (almost) exact phrase by putting the phrase in quotes:
"hello, john"
This will find both instances of hello john
and hello, john
.
I want to find only results that have the comma. Is this possible?
You can't. At least with Google.
There are more accurate search engines. I'd recomend http://symbolhound.com/ Not a great company, not a great design but fits better for your needs.
That's not quite possible because search engines (Google, Bing, DuckDuckGo) ignore comma individuality. One way would be to run search on search page via CTRL + F like: