Constant PIV_SLOT 

pub const PIV_SLOT: &str = r"^(piv:)?(9[acde]|8[2-9a-fA-F]|9[0-5])$";

A PIV slot as a step names it: 9a, 9c, 9d, 9e, or a retired slot by its key reference in hex, 82 to 95, optionally after piv:.