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

I have two columns like this:

and I want a new colmun like this:

How can I do this?

>Solution :

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;