Gitweb had a grep feature, probably the only thing we miss of it.
Gitblit supports only lucene search, which doesn't seem to scale for our repository (but needs more testing, I hear). Also: "I have no interest in implementing a regex search because the Lucene search fits my needs. If you would like to contribute a regex search I'd be happy to merge it." https://code.google.com/p/gitblit/issues/detail?id=87
https://blog.mozilla.org/webdev/2013/06/13/dxr-digests-the-firefox-codebase/ made me curious about DXR but the docs are too intricate (https://github.com/mozilla/dxr/issues/139).