General :: Selectively Remove Blank Spaces From String?

need all spaces between two letters or a letter and a number exchanged for an underscore, but all spaces between a letter and other characters need to remain. One example for clarity:

Input:

force -- lamin 90 [label]
active A -- generation [label]

needed Output:

force -- lamin_90 [label]
active_A -- generation [label]

I tried solving this with sed but obviously s/ /_/g does not work, nor does any s/[a-zA-z0-9] [a-zA-z0-9]/[a-zA-z0-9]_[a-zA-z0-9]/g , because you just can't do this...