Explain the differences of casex and casez over the case statement?

casex operator has to be used when both high impedance value (z) and unknown (x) in any bit  has  to  be  treated  as  a  don't-care  during  case  comparisons.  casez  operator  treats  (z) operator as a don't-care during case comparisons.

In both cases, bits which that are treated as don't-care won't be considered for comparison, that is, only bit values other than don't care bits are used in comparison. Wildcard character "?" can be used in place of "z" for literal numbers.