SQL & PL/SQL :: Alphanumeric Sequence Generator

Is there a function or a process to generate and maintain an alphanumeric sequence?

I need to be able to sequentially generate both a 3 and 5 byte case sensitive alphanumeric primary key that uses A-Za-z0-9 characters intermittently of which the 5 byte MAY be a subset of the 3 byte. e.g. the key can be 00000 or a3BD7.

so I need to be able to generate a new 5 char using 3 char base and if that value is not available, generate a unique 5 char value and allocate the substr(1,3) of that value backwards.

Ex1: unique customer location comes in - not a preexisting customer name (same name,different location address doesn't exist) --> select nextval.[5char] into v_long_c to insert into customer_loc table, and substr (v_long_c, 1, 3) for customer name table.

EX 2: unique customer location comes in. customer name exists in customer table with 3char val A39, but that location doesn't exist in customer_location table (5chars are location specific).

I need to take the A39 and generate new 5char for that unique location using that prefix (A39) if possible.

However, a number of a39-- already exist, though not all assigned to the same customer name (we're trying to keep them grouped together but that might not be logically feasible)

How do I select next a39||[2char] for that unique location - and if that value is not available (all 62*62 possibilities have been used for A39--), select nextval.[5charseq] into long_key.

I suspect someone out there knows the functions I can use to create this or has written a package to do just this; I suspect with enough time I could do it, but I don't currently have the time or knowledge to develop it within these deadlines. I thought it would be easier.

My short-sighted solution was to create a static table of all iterations of A-z0-9 5 char values, select one, mark it used and move on. Unfortunately 62^5 is a substantial number (913+million records) and that table took a LOT of space, causing my development server to groan and crack miserably. indexing on it takes a lot of space too (and trying to build multiple indexes exceeded database size). But without an index on the 3char field, selecting an available 5 code from it based on the customer_3char prefix took five minutes - much longer than the fraction of a second I need.