code: def _replace_envvar(s, _):
      
      e = s.split(":")
      if len(e) > 3 or len(e) == 1 or e[0] != "env":
          raise ValueError()
      elif len(e) == 2:
          # Note: this can/should raise a KeyError (according to spec).
          return os.environ[e[1]]
      else:  # len(e) == 3
          return os.environ.get(e[1], e[2])
docs: env:KEY or env:KEY:DEFAULT