Это пример:
create table lastword (
id int not null auto_increment primary key,
mytext varchar(250)
) engine = myisam;
insert into lastword (mytext) values
('this is my car'),
('how are you,doing'),
('how is your)health'),
('this is not(working');
select *,
substring_index(replace(replace(replace(mytext,',',' '),')',' '),'(',' '),' ',-1) as last
from lastword
+----+---------------------+---------+
| id | mytext | last |
+----+---------------------+---------+
| 1 | this is my car | car |
| 2 | how are you,doing | doing |
| 3 | how is your)health | health |
| 4 | this is not(working | working |
+----+---------------------+---------+
4 rows in set (0.00 sec)
Как видите, для выполнения вашей задачи вам понадобится много вложенных замен.