code: def nxm_field_name_to_ryu(field):
      
      if field.endswith("_W"):
          field = field[:-2]
      prefix = field[:7]
      field = field[7:].lower()
      mapped_result = None
  
      if prefix == 'NXM_NX_':
          mapped_result = _NXM_FIELD_MAP.get(field)
      elif prefix == "NXM_OF_":
          mapped_result = _NXM_OF_FIELD_MAP.get(field)
      elif prefix == "OXM_OF_":
          # no mapping needed
          pass
      else:
          raise ValueError
      if mapped_result is not None:
          return mapped_result
      return field
docs: Convert an ovs-ofctl style NXM_/OXM_ field name to
      a ryu match field name.