I read PM often, but don't have an account. I would think the following Algorithm would work best:

for all accounts
get general similarity of new with that account
multiply by number of recent posts of that account

most accounts where the result is over a thereshold must approve
accounts that must approve can increment the thereshold

for all accounts that have a regex set
if regex matches account must approve
accounts that must approve here can decrement the thereshold

to set a regex the regex must match the own accountname and it must not match any other accountname or must be approved from this other matching account