Select Cast Error - Mailing list pgsql-general

I have a field that is varchar(15) type and an example of data I'm working with is (PROJ-0001-06)

I can make these two select statements work but not together.

select cast((max(substring(test.test from 6 for 4))) AS INTEGER) + 1  FROM test;
select max(substring(test.test from 11 for 2)) FROM test;

I want this to work like this.

select cast((max(substring(test.test from 6 for 4))) as integer) + 1  FROM test where max(substring(test.test from 11 for 2));

List below is a better idea of what my table looks like and the result I need.

PROJ-0004-05

PROJ-0001-06

PROJ-0002-06

PROJ-0003-06

When I run my select statement I want to return the number 4.  The idea is that I need the next highest number in the middle but take in consideration that the highest trailing numbers take president.

There error I get is this  ERROR:  argument of WHERE must be type boolean, not type text