Регулярка -> обратная польская запись

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();

        Stack<Character> st = new Stack();

        String ans = "";
        for(int i = 0 ; i < s.length() ; ++i){
            char cur = s.charAt(i);
            if(!isDelim(cur)){
                if(cur == '('){
                    st.push(cur);
                }else if(isOperator(cur)){
                    while(!st.empty() && getPriority(st.peek()) <= getPriority(cur)){
                        ans += st.pop();
                    }
                    st.push(cur);
                }else if(cur == ')'){
                    while(!st.empty() && st.peek() != '('){
                        ans += st.pop();
                    }
                    if(!st.empty())st.pop();
                }else{
                    ans += cur;
                }
            }
        }
        while(!st.empty())ans += st.pop();
        System.out.println(ans);
    }

    static boolean isDelim(char  ch){
        return ch == ' ';
    }

    static boolean isOperator(char ch){
        return (ch == '*' || ch == '+' || ch == '.');
    }

    static int getPriority(char ch){
        switch(ch){
            case '*': return 1;
            case '+': return 2;
            case '.': return 2;
            case '(': return 3;
            default: return -1;
        }
    }
}
/*
a.(a+b+c*)*.c*
*/