프로그래머스 - 신규아이디

class Solution {
    public String solution(String new_id) {
 
        new_id = new_id.toLowerCase().replaceAll("[^a-z\\d\\-_.]*", "");
 
        new_id = new_id.replaceAll("\\.{2,}", ".");
 
 
        new_id = new_id.replaceAll("^[.]|[.]$", "");
 
 
        if (new_id.isEmpty()) {
            new_id = "a";
        }
 
        if (new_id.length() >= 16) {
            new_id = new_id.substring(0,15).replaceAll("[.]$", "");
 
        } else if (new_id.length() <= 2) {
 
            for (int i = 0; new_id.length() < 3; i++) {
                               String plusString = String.valueOf(new_id.charAt(new_id.length()-1));
 
                new_id = new_id.concat(plusString);
 
            }
        }
 
        String answer = new_id;
 
        return answer;
    }
}