How to split a string twice and add it to a linked list?

I tried to split a string and added it to a linked list each node in the linked list is a polynomial boundary . I tried this but it gave me a Dangling meta character exception , what was the wrong I did here ?

String s = "X^2+3x+5";
 LinkedList p_list = new LinkedList();

 s.toLowerCase();
 s.replace("*x", "x");
 s.replace("x^", "x");
 s.replaceAll("--","+");
 s.replaceAll("+-", "-");
 s.replaceAll(" ", "");
 String []  st = s.split("(?=[+-])");
 String [] st2 = new String[2];

 for(int i=0;i<=st.length;i++){
    if(st[i].contains("x")){
        st2=st[i].split("x");
        if(st2[0].length()== 0 && st2[1].length()== 0){
            p_list.addFirst(1,1);
        }else if(st2[0].length()== 1 && st2[1].length()== 0){
                p_list.addFirst(Integer.parseInt(st2[0]),0);
        }
    } else {
        p_list.addFirst(Integer.parseInt(st2[0]),Integer.parseInt(st2[1]));
    }


 }  


 p_list.printList();