How to derive a new colnum according to a given colnum in SAS?

Question

I have two columns like this:

subject	regnum
106001	2
106001	2
106001	2
106001	1
106001	1
106001	4
106001	4
106001	6
106001	6
106001	3
106001	3
106001	5
106001	5
106001	7
106001	7
106001	8

and I want a new column like this:

subject	regnum	regnum_new
106001	2	1
106001	2	1
106001	2	1
106001	1	2
106001	1	2
106001	4	3
106001	4	3
106001	6	4
106001	6	4
106001	3	5
106001	3	5
106001	5	6
106001	5	6
106001	7	7
106001	7	7
106001	8	8

How can I do this?

Answer 1

data have;
input subject regnum;
datalines;
106001 2
106001 2
106001 2
106001 1
106001 1
106001 4
106001 4
106001 6
106001 6
106001 3
106001 3
106001 5
106001 5
106001 7
106001 7
106001 8
;

data want;
   set have;
   by subject regnum notsorted;
   if first.regnum then regnum_new + 1;
   if first.subject then regnum_new = 1;
run;