What your going to have to use is the val() function, which gets a actual number value from a string, like, "3". But I don't understand your question. Do you want to check 2 characters in a string, and if they are numbers, add them up and place them at the end of the string? Or what? All I can give you right now is how to check a character in a string and see if it is a number 0-9.
While val will get a value from a string, if it finds that
the string has a letter, like "g", then it will return 0. Since you want to check for a number 0-9, we first have to find out if the text is a number or a letter.
'48-57 are the ascii codes for 0-9
IF ASC(MID$(string$, x, 1)) >= 48 and ASC(MID$(string$, x, 1)) <= 57 THEN
IF VAL(MID$(string$, x, 1)) >= 0 and VAL(string$, x) <=9 THEN
'This is a number 0-9, so do whatever you wanted to do with it
END IF
END IF
And, x is what part of the string you are looking at, for example: If x=3 and string$="abcd" then the program would be looking at "c". That is how the MID$ works.
Hope I wasn't to confusing.