Return the pattern used to check if a field name can be accepted by this
        dynamic field. Use a default one ('^fieldname_(.+)$') if not set when
        the field was initialized

def pattern(self):
        """
        Return the pattern used to check if a field name can be accepted by this
        dynamic field. Use a default one ('^fieldname_(.+)$') if not set when
        the field was initialized
        """
        if self.dynamic_version_of is not None:
            return self.dynamic_version_of.pattern

        if not self._pattern:
            self._pattern = re.compile('^%s_(.+)$' % self.name)
        return self._pattern