Alphanumeric Sequence

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I believe you should go for VBA approach since it's very hard to arrive it in using inbuilt worksheet functions...

I hope someone will come up with a solution

Just for the hell of it ...

This uses formulae only.

If you are in a masochistic mood you could expand the formula to eliminate the helper and the lookup table.

I think the next number in the sequence after RTC0009Y should be RTC000A0 but I'm getting RTC00100 with your method, Marcol

Try this assuming you want numbers to start in A2:

define a named range called alpha and in "refers to" box put this:

={0;1;2;3;4;5;6;7;8;9;"A";"B";"C";"D";"E";"F";"G";"H";"J";"K";"L";"M";"N";"P";"R";"T";"U";"V";"W";"X";"Y"}

then in A2 you can use this formula copied down

="RTC00"&INDEX(alpha,INT(ROWS(A$2:A2)/(31^2))+1)&INDEX(alpha,INT(MOD(ROWS(A$2:A2)/31,31))+1)&INDEX(alpha,MOD(ROWS(A$2:A2),31)+1)

No helper cells required

that should be good for 31^3-1 = 29790 rows