simplify archive regex

[labkode]

#2235 (review)

[defending1]

In #2235 (review) @ishank011 mentioned that the current regex is
/eos/user/[a-z]/[a-z]./\w.
Is there a place where the format of accepted folders is described in the docs?

I also see another problem about this expression.
A case that this expression would accept is
/eos/user/a/a;/3?
which is not allowed.