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