A pattern splitter can help you with this and I hope that you can complete the solution.
Reference: http://www.sqlservercentral.com/articles/String+Manipulation/94365/
Select *
from #tbl_data
CROSS APPLY (SELECT TOP 1 Item
FROM dbo.PatternSplitCM( PONumber, '%[0-9]%') s
WHERE Matched = 1
ORDER BY ItemNumber DESC) PO