SQL & PL/SQL

Hello everyone,

I have a small issue with regular expression. Look at the following example:

WITH T as (SELECT 'ABC' as a  FROM dual
           UNION ALL 
           SELECT 'ABC.Test' FROM DUAL
           UNION ALL
           SELECT 'XXX' FROM DUAL
)
SELECT a, 
       regexp_replace(a, '^(ABC)?(\.)?(.+)$','ABC.\3')
  FROM t

Don't pay attention to the regexp I did. It's wrong.
What I want is :
- when my string start with something else than ABC, it is replaced by the same string with ABC. in front of it.
- When the string starts with ABC. something, nothing changes
- When the string is ABC (without the final dot) the string doesn't change.

Any idea how I can do that using regexp_replace? (I'm using Oracle 10g)

Thanks