/^        # start of string
 0x       # 0x prefix
 [0-9A-F] # a hex digit
   {1,4}  #    1 to 4 of them
 $        # end of string
/xi       # x = allow spaces & comments in regexp
          # i = ignore case
[download]

or just /^0x[0-9A-F]{1,4}$/i

update: slight rewording to keep ww happy;)