Yes, this is the way. I thought it is possible to do it in one step, but this didn't work. As charley mention you need to do a second step for moving the data in the right position.
Doing this, you should maybe integrate a second custom column what indicate the completeness of your metadata so that you are aware of what metadata you have already finished. I do this with a yes/no column.
Chary, thanks for helping out.
Edit: Here an example how to make a quick replacement from one to an other field
I use your regex (?P<publisher>.{8}) to move the extracted information in the Metadata Publisher
File name: spe00095428 calibre User Manual — calibre User Manual.pdf
Publisher will become "spe00095428"
In my pic below I import the file two times. Then mark the imported books and click on "edit metadata" and select tab search and replace and select in 'Search field' publisher and in 'Destination field' your custom field (here my name is '#alt_title'. After performing the change you will find the data in the right place. You can do then a second edit to clean up the field publisher.
Be careful and test it on an example. When you do it with a bigger count of books, be sure you have select the books you want to change. You can't reverse these action.