What characters are allowed in the cashaddr ‘network type’ prefix

[Beakerboy]

The specification says to use the lowest 5 bits of each character in the Polymod function. Can the network string contain a ‘1’, whose lowest 5 bits are 17? What about an underscore, with bits = 31? What about one of the signal characters like ‘line feed’? If there is a specification or RFC for this, please add it to the specification document.