Many challenges don't specify what characters the input consists of, so I often assume the input will only consist of printable ASCII chars. Is my assumption correct?
Note that this mostly concerns string-related challenges.
String-based challenges should always specify the valid character range, much like numeric challenges should specify the range of valid input numbers. It's even the very first thing in the string tag wiki. I propose putting challenges on hold as unclear until the character set has been clarified.