Return a reference to the non‐modifiable current (trailing) delimiter (maximal sequence of one or more delimiter characters containing at most one hard delimiter character) in the input string. The returned reference remains valid so long as the underlying input is not modified or destroyed ‐‐ irrespective of the state (or existence) of this object. The behavior is undefined unless the iteration state of this object is initially valid, or if the underlying input has been modified or destroyed since this object was most recently reset (or created).