uuid() returns a character of length 1 containing exactly 36
characters: 32 hexadecimal characters and 4 hyphens (used as separators).
uuid_raw() returns a raw vector of length 16.
uuid_is() returns a logical vector having the same length as x. It
checks whether its elements are valid version 4 (variant 1) UUIDs or
not. It returns FALSE for any other kind of UUID.