You can use this trick to implement before insert trigger in sybase ASE:
create trigger myTrigger on myTable
rollback -- you rollback your insert
-- here some before insert action
insert into myTable select * from inserted -- and here your make insert again
but, I'm not sure then it's useful for all situations
That is incorrect, Sybase ASE has only "before" triggers, that is, the trigger is fired after you INSERT/UPDATE/DELETE starts (when you submit the statement to the server) and before the rows are INSERTed, within the context of your transaction. The trigger code inspects a virtual table "inserted", which contains the rows you are attempting to INSERT, which you can manipulate (eg. DELETE from inserted); and has full capability on the rest of the database for verifying the attempted INSERT.