/* package whatever; // don't place package name! */
 
class Main{
  static boolean c(String i){
    return i.matches("(?x)(?:a(?=a*(\\1?+b)))+\\1");
  }
 
  public static void main(String[] a){
    System.out.println(c("ab"));
    System.out.println(c("aabb"));
    System.out.println(c("aaabbb"));
    System.out.println(c("aaaabbbb"));
    System.out.println(c("aaaaabbbbb"));
 
    System.out.println(c(""));
    System.out.println(c("a"));
    System.out.println(c("b"));
    System.out.println(c("aa"));
    System.out.println(c("ba"));
    System.out.println(c("bb"));
    System.out.println(c("aaa"));
    System.out.println(c("aab"));
    System.out.println(c("aba"));
    System.out.println(c("abb"));
    System.out.println(c("baa"));
    System.out.println(c("bab"));
    System.out.println(c("bba"));
    System.out.println(c("bbb"));
    System.out.println(c("aaaa"));
    System.out.println(c("aaab"));
    System.out.println(c("aaba"));
    System.out.println(c("abaa"));
    System.out.println(c("abab"));
    System.out.println(c("abba"));
    System.out.println(c("abbb"));
    System.out.println(c("baaa"));
    System.out.println(c("baab"));
    System.out.println(c("baba"));
    System.out.println(c("babb"));
    System.out.println(c("bbaa"));
    System.out.println(c("bbab"));
    System.out.println(c("bbaa"));
    System.out.println(c("bbab"));
    System.out.println(c("bbba"));
    System.out.println(c("bbbb"));
  }
}

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