You cannot swap the operands. For example, with the given numbers, you cannot get the expression (1 + 3) * 2 = 8.

You need to add a digit only when it's needed, like from 9 to 10 or 99 to 100. In the cases you mentioned, there is no need to add one, only keep the same amount of digits. Not a kata issue.

Attention: If the number has leading zeros the amount of digits should be considered.

You expect 'foobar00' to be incremented to 'foobar01', then you expect 'foobar099' to be incremented to 'foobar100'. 'foobaar001' should be incremented to 'foobar002'. Retain '0's in some and lose them in some other cases. Do not randomly design Katas just because you've to. Let there be some logic.

PS: I used Python3

I think the expression: (a + c) * b, was not taken into consideration while designing this Kata. Removing it from your array of possible expressions should fix it if you're using the same approach as mine.