Left Function in SQL Statement

I am trying to return the LEFT 14 characters known as PO_NBR.  The string starts out as numbers, characters and other assorted **** so I am trying to clean this up by using a regular expression.

Here is what I have so far: 

SELECT REGEXP_REPLACE(UPPER(PO_NBR),'[^0-9A-Z]','') AS PO_NBR

FROM DTF_BUDGET_DETAIL

GROUP BY PO_NBR

When I try to do this:

SELECT LEFT(REGEXP_REPLACE(UPPER(PO_NBR),'[^0-9A-Z]',''),14) AS PO_NBR

FROM DTF_BUDGET_DETAIL

GROUP BY PO_NBR

I get an error "ORA-00904 "LEFT: invalid identifier"

Any ideas on how I might do this?

Thank you,

Scott