Some questions on charsets

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I don't quite get how the charsets work, so I have a few questions.

If I write "?l?l?l?l?l?l?l?l" as charset in oclHashCat, then I have defined a charset of 8 characters, ranging from aaaaaaaa-zzzzzzzz, right?

But, if I write "?l?l?l?l?l?l?l?d" as charset, then I have defined it as 8 characters, ranging from aaaaaaa1-zzzzzzz9, right? And so on..

If what I've written above is correct, how can I define a charset on 8 characters, a-z, 0-9? 9 characters and a-z, 0-9 and so on..

Thanks in advance for answers.