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Their latest code search thing [1] is pretty powerful already imo.

Sweet! Joined the waitlist.

Alternatively, until that becomes available: https://sourcegraph.com.

Any comparison with grep.app?

With sourcegraph, I am able to execute language context aware searches. It goes far beyond just grep-like matching and regular expressions. This page has more details: https://docs.sourcegraph.com/code_search/reference/language.

There are other awesome features in sourcegraph, for example searching diffs, searching within specific branch or tag.