Find remaining serial numbers left in Alphanumeric range

Question

Can anyone help me with this.
Say there are two alphanumeric sequences 1234A1B  And 1234B1B . The no. of series between them are to be found.
Say between 1234A1B and 1234B1B there are 26 numbers like 1234A1C,1234A1D,…1234A1Z,123B1B..In this case the output would be 26.The number of series in between are to be calculated.
The length of both the sequences is less than 18 and contain alphabets and numbers in them in random fashion e.g 1234A3D -1234C7E(The number series existing  between these two numbers should be counted).
Thanks in Advance
Rijish

Answer 1

Rijish,
First, a question.  In your example, the "from" and the "to" are both 7 characters, and you are considering only 7-character values between them.  But is not 1234A1BA (8 characters) alphabetically between 1234A1B and 1234A1B?  Or are you regarding the alphanumerics somehow as numbers?
Second, another question.  Have you mistyped your example?  You seem to imply that 123B1B comes after 1234A1Z.  Do you mean that 1234B1B comes right after 1234A1Z?  That would be confusing too.
Third, another question.  The characters allowed are A-Z and 0-9 only (36 in all)?  And their order (lowest first) is 0123..89ABC...XYZ?
Fourth, a warning.  The number of such serial numbers possible is rather large.  It is, I think, a 27-figure number.  So you need to consider the data type for holding  the answer to your calculation.
I may be able to help more if you can clarify your meaning.
John