By default, a value or transition bin definition can specify 4-state values.
When a bin definition includes an X or Z, it indicates that the bin count should only be incremented when the sampled value has an X or Z in the same bit positions.
The wildcard bins definition causes all X, Z, or ? to be treated as wildcards for 0 or 1 (similar to the ==? operator).
For example:
wildcardbins g12_16 = { 4'b11?? };
The count of bin g12_16 is incremented when the sampled variable is between 12 and 16:
1100 1101 1110 1111
program main;
reg [0:3] y;
reg [0:3] values[$]= '{ 4'b1100,4'b1101,4'b1110,4'b1111};