1、匹配两个字符串之间的内容
NSString *startStr = @"aaaa";
NSString *endStr = @"bbb";
NSString *str = @"aaaa8888999bbbkdkdkdkd";
NSString *regex = [NSString stringByFormat:@"%@[\s\S]*?%@",startStr,endStr]; //正则规则