Method string = string.replace ("a", "b"); will replace all letters. But what if only the freestanding ones need to be replaced? That is, we change "a" , but "aa" remains to itself. Is there a way to somehow implement this?

Answer 1, authority 100%

Use a regular expression: (? & lt;! a) a (?! a) , while the adjacent characters “a” like “aaaa” are ignored.

• (? & lt;! a) any character not “a” before character “a”
• a the character “a” itself, which we replace

• (?! a) any character not “a” after the character “a”

  Sample code how to use

  public static void main (String [] args) {
    String testString = "amaaaamaaaamkkjhatapa";
    System.out.println (testString.replaceAll ("(? & Lt;! A) a (?! A)", "."));
  }
  

Console output .maaaamaaaamkkjh.t.p.