I've been told that I can parse the subject line of a case using flow and process builder but I really have no idea where to start. Can someone give me an outline of the steps I would take? I've done some investigating in flow but the only thing I can see that may work would be to use a loop variable but I don't know if I'm even heading in the right direction. 

Here's what I'm hoping to accomplish: 
We have cases that all come in from the same email address. The email address is not associated with any of the contacts on the accounts however they have the correct account number in the subject line of the case. I need to assign the case to the correct account based on the account number in the case subject line. All of the account numbers are six characters long. 

Here's an example of what the subject line would look like: "123456 Backups complete on "