java 将字符串变成小写 单引号内的字符串大小写不变

public static void main(String[] args) {
        String str = "asdfFFFSSDAF'aaaaAAA','132213'";
        System.out.println(str.toLowerCase());
        String newStr = "";
        boolean flag = false;

        for (int i = 0; i < str.length(); i++) {
            if ("'".equals(str.substring(i, i + 1))) {
                flag = !flag;
            }
            if (flag == false) {
                newStr = newStr + str.substring(i, i + 1).toLowerCase();
            }else{
                newStr = newStr + str.substring(i, i + 1);
            }
        }
        System.out.println(newStr);
    }

输出：

asdffffssdaf'aaaaaaa','132213'
asdffffssdaf'aaaaAAA','132213'