Perl match problem

If I want to find the text wheather match the RD[$j] $j is the any
number
between -25 to 25, can I write the following sentence :
for ($j=-25;$j<=25;$j++)
if($parts[0] =~ /RD[$j]/)

I am not sure the is expression, since the code can not work. Thanks
for any
comments

.