Not that I know of.
You have to use string manipulation functions to achieve your goal.
Right(string, length) is a function that returns a specified number of characters from the right side of a string. This would chop of the prefix.
If DON(MC) is always the prefix, then the following function should take care of it.
response = "MC" & right(mystring,12)
If you don't know the prefix (or the final length of the string) then use the function, LEN. Len(string) is a function that returns the number of characters in a string.
If complicated demands thwart these plans, you can also get search for a character within a string.
(InStr([start, ]string1, string2[, compare])
This is the day the Lord has made, I will rejoice and be glad in it.