question of regular expression

[Stephen]

Actually, I don't know which newsgroup should I ask for this question?
I am now using org.apache.regexp for regular expression in Java. I
want to search the unicode characters (i am afraid the others can't
understand the language of the unicode, so I still try to use English
for my example, assume all the following words are unicode)
e.g.
I want to search the words with the following possibilities:

AB or C or E or ST

how to I write the expression?

I have tried this:

(AB|C|E|ST), however, it also matches when there is B and T only....
(while only AB or ST allows to be matched)......

thx!