Use of replace,replaceAll,matches, . and ^ are discussed below.

 String myString = "Its going to rain today";
 System.out.println(myString);
 String yourString = myString.replace("Its", "Is it");
 System.out.println(yourString);

. is a wild card for any character. so all characters will be replaced by . in below example

  String alphanumeric = "abcdefghiiiiiiiiiiik991";
  System.out.println(alphanumeric.replaceAll(".", "Y"));

Carrot boundary matcher which we can type using shift + 6. It basically means start looking at the start of the String due to ^. and match the expression exaclty

 System.out.println(alphanumeric.replaceAll("^abcdef", "ABC")); //This will replace abcdef in above String with ABC

Please note that ^ replaces only if starting of String is exact match to abcdef.So the second occurence of abcdef will not be replaced in below example

   String newAlphanumeric = "abcdefghijiiabcdefiiiiiiiik991";
   System.out.println(newAlphanumeric.replaceAll("^abcdef", "ABC"));

Carrot character and matches.The below will return false as for the matches class the entire String has to match not just the beginning of the String

   System.out.println(alphanumeric.matches("^abcdef"));

The below will return true as we are matching the entire String

  System.out.println(alphanumeric.matches("abcdefghiiiiiiiiiiik991"));