create index with substr function

Hi All,

I have a table in my postgres:
Table: doc
     Column     |            Type             | Modifiers 
     ---------------+-----------------------------+-----------
 doc_id          | bigint                      | not null
 comp_grp_id | bigint                      | not null
 doc_type      | character varying(10)| not null
 doc_urn        | character varying(20)| not null

I want to create an index on doc_urn column with using substr function like this:
CREATE INDEX idx_doc_substr_doc_urn ON doc USING btree (SUBSTR(doc_urn,10));

but there is an error:

ERROR:  parser: parse error at or near "10" at character 68

what's wrong for this SQL? As I have found some reference on the internet, I can't find anything wrong in this SQL.

Thanks
Ray