What's the regular expression for splitting text between small and capital letter?

Question

Regular Expressions

Good afternoon.
There is a text in which the names of the streets are "glued together", for example: Velyka Zhytomyrska
Please tell me the regular expression to separate "аЖ" with a space, i.е. find two characters (small letter and capital letter) and put "space" between them

Answer 1

preg_replace regular expression regexr.com/3bvkr and replace with
"$1 $2"

Answer 2

A special case of the previous answer for php and unicode + the letter Y:

preg_replace('/([а-яё])([А-ЯЁ])/u', '$1 $2', 'ВеликаЖитомерська')