Sorry if this overlaps already-posted code, was busy for a long while before I was able to get back to working on this:

SELECT string, CASE WHEN next_to_last_space = 0 THEN ''

ELSE SUBSTRING(string, LEN(string) - next_to_last_space + 2,

next_to_last_space - last_space) END AS value

FROM (

VALUES('INV CRT IS15000467 1'),

('INV CSH 144934 1'),

('INV CSH IS15000134 1'),

('INV CSH IS15000442 10'),

('INV CSH IS15000442 xx q rrrrrr e10'),

('INV ERR'),

('INVE')

) AS test_data(string)

CROSS APPLY (

SELECT CHARINDEX(' ', REVERSE(string)) AS last_space

) AS assign_alias_names1

CROSS APPLY (

SELECT CASE WHEN last_space = 0 THEN 0

ELSE CHARINDEX(' ', REVERSE(string), last_space + 1) END AS next_to_last_space

) AS assign_alias_names2

Edit: Split the "ELSE" code in the main query to separate lines.