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Uh, it's not a "reddit hack", it's a documented feature of Google Search -- limiting results to ones from a certain domain. You can use site:<domain.tld> for anything Google indexes.
It's particularly useful for finding immediate links to obscure parts of government websites or, say, looking through old press statements.
You can also use inurl: to find results whose URL contains the string that follows. There's all sorts of interesting things you can do to narrow results from Google, most of it is surprisingly intuitive. https://support.google.com/websearch/answer/2466433?sjid=15038670024975993899-NA