How to increment a string?

I have a TextBox that I want the user to be able to input a string consisting of 1-3 letters followed by 1-2 numbers.  For example "A1" or "AAA11".

How do I increment this string by 1 so that I can use the new value?

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Perhaps

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AA11 + 1 = AA12 or AA111 ??
AA99 + 1 = ?

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first you have to separate the numbers 

then add 1

the for loop is searching for the last letter. -> NumberStart+1 is the first number

then the whole number will be separated

and add 1

 (you can replace the TextBox by a Variable)

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