To compare GitHub to Sourcegraph search, here is that same query on Sourcegraph.com (which is Sourcegraph Server running for all open-source code on GitHub):
https://sourcegraph.com/search?q=repo:Kurento/+DISABLE_LIBRA...
It works as expected (and as the SO poster wanted)! It shows desired results that GitHub search does not.
Regexps are also supported...give it a try!