How to break sticky sentences with regular expressions? wordWord -> word. Word?

Question

Regular Expressions

There is text, but in it some sentences stuck together with the following:
Beginning middle end tsN beginning middle end

In fact, the task is to find words using regular expressions in the middle of which there is an uppercase character. And before it insert a dot and a space

Answer 1

=REGEXREPLACE(A1;"([а-яё])([А-ЯЁ])";"$1. $2")
Demo table

Answer 2

find [a-za-z ]+([A-ZA-Z][a-za-z]+[^\s])
replace dot space $1