Construye la expresion

def result(self):
        """
        Construye la expresion
        """ 
        field = re.sub(REGEX_CLEANER, '', self.field_name)

        if self.alias:
            alias = re.sub(REGEX_CLEANER, '', self.alias)
            return "%s AS %s" % (field, alias)
        else:
            return field