SQL & PL/SQL

I am trying to implement a conditional unique constraint on this table.

 
UID   VALUEA   VALUEB   VALUEC   EFF_DATE   TRM_DATE
  2    105       105      32     3/15/07    3/21/09
  3    142        21      86      3/7/07    7/25/09
  5    105       105      32      2/1/07    3/10/07
  7    142        21      86      3/5/07    6/12/09

The constraint would block UID 7 but not 5.

The technique I am trying to use is to create a unique index based on a function. Much like this one:

 
CREATE UNIQUE INDEX ITEM_U01 ON ITEM
(CASE ITEM_TYPE WHEN 'BOX' THEN SHORT_NAME ELSE NULL END)

This ensures that no items of the type "BOX" have the same name.

ps.
What are the tags to keep my code formatted? I can not remember.