The simple answer:
It matches any capitalized word that isn’t at the beginning of the line (checked using a negative look-ahead and a word boundary).
But your requirements are sketchy…
John and I are good friends.
I. Is that really what you want?
And it’ll only work for the first sentence on a row. Also a line starting with a non character will fail. E.g
-I like John, she said.
“This is a quote.”
solved Find a singular word that has a capital letter [closed]