Simple Question for M_R

[atlanta]

Hi i don't know if im doing this right so i need a little help

i wanted this
http://mydomain.com/test.php?sn="input"

go to this
http://mydomain.com/aim/"input".png

i put this in but its not working
RewriteRule ^aim/([a-z]+)  test.php?sn=$1 [L]

and "input" being what ever the user puts in i also what the reg expression to be able to use alpha and numerical unlike the above.

Thanks Alot