I have rows that look like this:

junkjunk junk more junk aa1234 still more junk and even more
junk
this is also junk aa4567 and so is
this, just junk
junk aabcde junk junk junk junk

And I want to remove all text and spaces except the 6 character string that
starts with aa.
How can I do that? I've looked at Left and Right and other functions but I
think I'm just too dense.