code: public JsonUnflattener withLeftAndRightBrackets(char leftBracket,
        char rightBracket) {
      isTrue(leftBracket != rightBracket, "Both brackets cannot be the same");
      isTrue(!Character.toString(leftBracket).matches(illegalBracketsRegex()),
          "Left bracket contains illegal chracter(%s)",
          Character.toString(leftBracket));
      isTrue(!Character.toString(rightBracket).matches(illegalBracketsRegex()),
          "Right bracket contains illegal chracter(%s)",
          Character.toString(rightBracket));
  
      this.leftBracket = leftBracket;
      this.rightBracket = rightBracket;
      return this;
    }
docs: A fluent setter to setup the left and right brackets within a key in the
  flattened JSON. The default left and right brackets are left square
  bracket([) and right square bracket(]).
  
  @param leftBracket
  any character
  @param rightBracket
  any character
  @return this {@link JsonUnflattener}