def resolve_regex_field_names(self, schema):
        """Resolve string(s) (regular expression(s)) that were passed into 'fields' and 'timestamp_field' parameters
        """
        self._fields = {k: self.convert_fields(schema, self._fields[k]) for k in self._fields.keys()}
        converted_ts_field = self.convert_fields(schema, [self._timestamp_field])
        if len(converted_ts_field) > 1:
            raise ValueError("timestamp_field was matched to more than one unischema field")

        self._timestamp_field = converted_ts_field[0]
Resolve string(s) (regular expression(s)) that were passed into 'fields' and 'timestamp_field' parameters
        
petastorm/ngram.py