This recently came up in another thread so I thought people would be interested.
By default, fanfics downloaded with FanFicFare have a
url: identifier. This works perfectly fine. However, I also have
ao3: and
ffnet: identifiers, for Archive of Our Own and FanFiction.net respectively.
This has two advantages, both seen
in this screenshot:
1. If a story is available on both websites, it means I can add both of them.
2. It looks nicer in Book Details.
However, note: FanFicFare checks for a
url: identifier when updating fanfics, so not having one can slow things down. I tend to keep it on incomplete fics, then remove it when the fic is finished.
So, if you like the sound of this, here's a quick tutorial on setting them up.
Preferences > Look & Feel > Book Details > Create rules to convert identifiers into links.
Now now when you copy a story URL from one of those two sites, you can open the metadata editor and use the "paste identifiers" button to get a nicely-formatted one.

But that's kind of a pain to do if you have more than a dozen fics.
So two options here.
First Option: Action Chains
Action Chains. Attached is "ConvertFanficIDs.zip", which you
import into Action Chains.
Two caveats:
1. For this to work, the publishers have to be exactly "Archive Of Our Own" or "FanFiction.net". However, you can edit the templates within the chain if you use something else.
2. Do
not use these on FanFicFare anthologies. The
url: is usually not a work-specific one, so you'll get odd results.
For those curious, here's the templates that do the work:
Spoiler:
Code:
program:
publisher = $publisher;
ids = $identifiers;
u = select(ids, 'url');
if
publisher == 'Archive of Our Own'
&& u
then
n = re(u, '^https://archiveofourown.org/works/(\d+)$', '\1');
ids = list_union(ids, strcat('ao3:', n), ',')
fi;
ids
Code:
program:
publisher = $publisher;
ids = $identifiers;
u = select(ids, 'url');
if
publisher == 'FanFiction.net'
&& u
then
n = re(u, '^https://www.fanfiction.net/s/(\d+)/(\d+)/.*', '\1');
ids = list_union(ids, strcat('ffnet:', n), ',')
fi;
ids
Second Option: Search & Replace
This is what I used before the Action Chains plugin existed. Note that unlike AC, it
doesn't check the publisher. (When I got a bit careless, I sometimes ended up with ids like
ffnet:https://archiveofourown.org/works/xxxx 
)