Is it a prime? w/o math
Ruby: 52 - 10 = 42

Using a variation of that famous prime-matching regex.

puts ?_*gets.to_i=~/^(_|(__+?)\2+)$/?"Not!!":"Prime"


Just to be clear: ?_*gets.to_i is a string operation that appends "_" to itself n times, where n is the input number. As I see it no string lengths are compared, so that should satisfiy the 10 character bonus criterium.