These are considered Unicode properties.
The Unicode property \p{L} — shorthand for \p{Letter} will match any kind of letter from any language. Therefore, \p{Lu} will match an uppercase letter that has a lowercase variant. And, the opposite \p{Ll} will match a lowercase letter that has an uppercase variant.
Concisely, this would match any lowercase/uppercase that has a variant from any language:
AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz