code: def _expand_formatting_rule(rule, national_prefix):
      
      if rule is None:
          return None
      if national_prefix is None:
          national_prefix = u("")
      rule = re.sub(u(r"\$NP"), national_prefix, rule)
      rule = re.sub(u(r"\$FG"), u("$1"), rule)
      return rule
docs: Formatting rules can include terms "$NP" and "$FG",
      These get replaced with:
       "$NP" => the national prefix
       "$FG" => the first group, i.e. "$1"