If you're building strings of random lengths like that, you're going to test the same three-character codes a lot of times if you let it run for any length of time. Usually it's better to pick a fixed length. The length to pick depends on how specific a result you are looking for. A good rule of thumb is to use one or two characters more than the string length you are searching for. If you are using any more complex conditions for your search, it gets trickier, and you might need to either go through the combinatorial maths to figure out an optimal length, or just pick a sufficiently long one.