sql server parse column and insert into another column

I have a column with both chars and numbers that are separated by an Underscore

Ex: PI (column Name) = ID_32,ID_43,ID_03

I also created a new column called UniqueColumn. In this column I just want the numbers that are in the PI column

therefore it should look like this: UniqueColumn=32,43,03

My code thus far:

UPDATE table
  SET UniqueColumn = RIGHT(PI,LEN(PI)-CHARINDEX('_',PI));


select top 10 dbo.table.UniqueColumn from dbo.table;

via Chebli Mohamed