code: def envRegisterFilter(self, name, attr_regex = '^\w+$', default = True):
          
          attrs = {}
          for prefix in ('include', 'exclude'):
              key = "%s_%s" % (prefix, name)
              val = self._env.get(key)
              if val:
                  attrs[prefix] = [attr.strip() for attr in val.split(',')]
              else:
                  attrs[prefix] = []
          self._filters[name] = MuninAttrFilter(attrs['include'], attrs['exclude'], 
                                                attr_regex, default)
docs: Register filter for including, excluding attributes in graphs through 
          the use of include_<name> and exclude_<name> environment variables.
          The value of the variables must be a comma separated list of items. 
          
          @param name:       Name of filter.
                             (Also determines suffix for environment variable name.)
          @param attr_regex: Regular expression string for checking valid items.
          @param default:    Filter default. Applies when the include list is not 
                             defined and the attribute is not in the exclude list.