How to find "intersection" of two strings?
In the absence of any other examples or formal criteria for how matching ought to be performed, I've just given an example here of the simplest way to obtain the difference between two strings:

    set A to "I like cherries, apples, berries"
    set B to "I like cherries, apples, berries, cookies"


    set the text item delimiters to {null, A}

    return text items of B as text --&gt; ", cookies"


This works if one string is contained entirely within (i.e. is a substring of) the other, as was the case with your given strings.  If string A were to be `"But I like cherries, apples, berries" or "I like cherries, apples, berries.", the above method will not work.

It also won't work if string B contains the word "null" where string A does not.