I'm going to agree with what a few others have said and say that you not worry about repeating patterns. Even if a password was ztztztzt, its not anymore likely to be cracked than any other string. What you really have to worry about is passwords that would be in a common dictionary file.

Another thing to consider is every word in a certain dictionary file, plus either appending any two characters, or prepending any two characters. The addition of two characters is not a far stretch for a password cracker to get.

A good idea might be to get a really good dictionary file, then search the password for any substrings that are one of those dictionary words. Then perhaps you could find out how many characters in the password are not said substring, and have some constant number that you check this against.

If you need a strong password policy, I suggest forcing users to use at least ten characters, and suggest that they choose a line from a favorite song, then grab the first or second letter of each word to make up there password. This makes it secure and easy to remember. Easy to remember is important because if your users are writing their passwords down everywhere, there not exactly secure.