code: def _select_options(pat):
      
  
      # short-circuit for exact key
      if pat in _registered_options:
          return [pat]
  
      # else look through all of them
      keys = sorted(_registered_options.keys())
      if pat == 'all':  # reserved key
          return keys
  
      return [k for k in keys if re.search(pat, k, re.I)]
docs: returns a list of keys matching `pat`
  
      if pat=="all", returns all registered options