I can't seem to get this substring extraction to work right. In a normal extraction you can use a variable for the number you're extracting from:

set variable=12345
set X=%variable:~3,-1%

Where 4 would be the desired number extracted. I run into my problem when trying to repace "3" and "1" with variables:

set variable=12345
set y=3
set z=1
set X=%variable:~%y%,-%z%%

I can get them all to display their respective values by playing with parentheses, but the expression is never able to set X=4 as desired.

My question is: How do you set proper variables within a variable substring extraction.