private static String replaceMultipleCharactersInString(String original_str, int index, char replace_value) {

        StringBuffer myString = new StringBuffer();

        for(int i = 0; i < original_str.length(); i++) {

            myString = (i % index == 0) ? myString.append(replace_value) :
                    myString.append(original_str.charAt(i));

        }

        return myString.toString();
    }