I'm not quite sure what you are trying to accomplish with this regular expresion, the first part looks like it is saying that the first 6, 8 characters cannot be a number, but the second part looks to allow any 6, 8 character string that use capital or lower case letters or numbers. It looks like the first part is contradictory to the second part.
A regular expression to allow a 6, 8 character string of upper and lower case letters as well as numbers would look like:
/^[0-9a-zA-Z]{6,8}$/
if this is not what you are after, please provide some detail.