code: def get_base_branch(cherry_pick_branch):
      
      prefix, sha, base_branch = cherry_pick_branch.split("-", 2)
  
      if prefix != "backport":
          raise ValueError(
              'branch name is not prefixed with "backport-".  Is this a cherry_picker branch?'
          )
  
      if not re.match("[0-9a-f]{7,40}", sha):
          raise ValueError(f"branch name has an invalid sha: {sha}")
  
      # Validate that the sha refers to a valid commit within the repo
      # Throws a ValueError if the sha is not present in the repo
      validate_sha(sha)
  
      # Subject the parsed base_branch to the same tests as when we generated it
      # This throws a ValueError if the base_branch doesn't meet our requirements
      version_from_branch(base_branch)
  
      return base_branch
docs: return '2.7' from 'backport-sha-2.7'
  
      raises ValueError if the specified branch name is not of a form that
          cherry_picker would have created