Question

How to convert "HelloWorld" to "Hello World"? The splitting has to take place based on The upper-case letters, but should exclude the first letter.

Answer 1

String output = input.replaceAll("(\\p{Ll})(\\p{Lu})","$1 $2");

This regex searches for a lowercase letter follwed by an uppercase letter and replaces them with the former, a space and the latter (effectively separating them with a space). It puts each of them in a capturing group () in order to be able to re-use the values in the replacement string via back references ($1 and $2).

To find upper- and lowercase letters it uses \p{Ll} and \p{Lu} (instead of [a-z] and [A-Z]), because it handles all upper- and lowercase letters in the Unicode standard and not just the ones in the ASCII range (this nice explanation of Unicode in regexes mostly applies to Java as well).