Using Git, how could I search within all files in all local branches for a given string?

GitHub specific: is it possible to perform the above search across all GitHub branches? (There are several remote branches on my remote GitHub repository that ideally I wouldn't have to bring down for this search...)

You can do this on a Git repository:

git grep "string/regexp" $(git rev-list --all)

GitHub advanced search has code search capability:

The code search will look through all of the code publicly hosted on GitHub. You can also filter by:

• the language: language:
• the repository name (including the username): repo:
• the file path: path: