Register Array position location

I define a register group as :

reg[63:0] temp;

Can I use a variable of "reg[6:0] a" to the temp as temp[a:0] ?

when ""reg[6:0] a" is input port,it may work.But when the "a" is calculated in the design,i.e.it's not a ration,how the expression turns out ?

thank u!