i think you will run into an error, you can use the “Matches activity” parse in your variable (String1) as an input, and use the above expression put it inside " " and then create an output variable from the matches property, your varibale could be for example “Matches_Results” that will return a collection, you can then use Matches_Results(0).ToString that will give you the first alphabet and so on.
so you can use a for each activity and parse in Matches_Results insite the for each you can use write line and say “Item.ToString” that will print out all the alphabets (A,B,C) then you can decide what to do with them.