Index creation question for expression (col1 || '-' || col2)

Greetings,

A question about creating index for the following expression.

CREATE INDEX idx_t1 ON test (col1 || '-' || col2);

May I know is it possible and how I may create index for the expression 
such as "col1 || '-' || col2" for a table please?

Thanks a lot,
Emi