如何计算NSString对象中的非字母数字字符数？

NSString *str = @"Hello123!@#";
NSInteger count = 0;

for (int i = 0; i < str.length; i++) {
    unichar character = [str characterAtIndex:i];
    if (!((character >= 'A' && character <= 'Z') ||
          (character >= 'a' && character <= 'z') ||
          (character >= '0' && character <= '9'))) {
        count++;
    }
}

NSLog(@"非字母数字字符数：%ld", count);