You can use the following commands before or after entering your search term:
from:me to search for messages only created by the current user.
from:user.name to search for messages created by a specific user. The username entered must be the format without spaces (i.e. "john.doe" and not "John Doe". If you want to search for anytime a user was mentioned, simply search for their username.
has:star returns messages that are starred by the current user.
is:pinned or has:pin returns messages that are pinned in the current channel.
has:url or has:link returns messages that contain a link.
before:dd/mm/yyyy, after:dd/mm/yyyy and on:dd/mm/yyyy return message that were created before, after or on the provided date. You can also use dashes dd-mm-yyyy or dots dd.mm.yyyy instead of slashes.
order:asc sorts messages by ascending timestamp. You can also use order:ascend or order:ascending
order:desc sorts messages by ascending timestamp. You can also use order:descend or order:descending.