Test Price 100______0
(.+) (.+)  (\d{3}) (\d)   <-- this matches

Test Price 100________
(.+) (.+)  (\d{3}) (\d)   <-- this doesn't match, there is no final (\d)

Test Price 800________
(.+) (.+)  (\d{3}) (\d)   <-- also doesn't match

        Note the success ----.  and failure points
                             |  |
                             v  v
Find: ^(.+) (.+) (\d{3})(?:(\d)|$)
Replace: \0 (?4:too expensive:affordable)

Color_ 1__ green
(Color \d) (.+)   <-- match

Color_ 2__ blue
(Color \d) (.+)   <-- match

Color_ 3__ red
(Color \d) (.+)   <-- match

The available colors are either green, blue or red.
(Color \d) (.+)   <-- no match anywhere on line