You can't use a varible length expression (something with a * or + or even ? as a modifier for a character to find) in a look-behind. You'll have to find the entire string instead. You can use markin subexpressions, if it will help, and you may need to use a second search for just the capitalized word to undo wahtever you did with the first one.
If, for instance, you are trying to capitalize the word inn, you could search this way
and change to
This will leave you first word alone, replacing it with itself.
The change to shoud be
$1 followed by whatever changes you want to make to spaceinn
Thanks very much. I searched the full string as you suggested and then just made the change (from inn to Inn) manually.