Whenever I work on huge codebase (think 1M+ lines of code), I always reach for Russ Cox's codesearch https://github.com/google/codesearch. It requires indexing the codebase first, which takes 15 minutes or so, but after that searches are instant.
I reach for Hound-search[0] (originally Etsy Houndd) that uses Russ Cox's "Regular Expression Matching with a Trigram Index"
I had even made self-serve hosting for it but didn't put much effort into monetizing or otherwise promoting it.