I'm not sure why I'm struggling with this as it seems a simple enough idea.

My folder names always include the year:

Blah_blah_blah_2015_Audio
Blah_blah_blahsomething_2016_Audio

I want to read this year from the folder name, and set it to the year field of the contained MP3s

I have set variable 3 to the folder name using Get Property

I am trying to use "Find all matches for regular expression" to set the result to the Year field (see pic).

Doesn't appear to work this way, I must be thinking about this completely wrong!

I'm using a fairly simple regex that should match a block of 4 digits that starts with a 2 (you know, so I don't have to update it again for a thousand years) and to minimise the possibility that the title may contain some other 4 digit block.

You got it. The statement is a replace not an extraction.

You could do the following:

Set Variable 0 to "\~"
Find first match for regular expression "(2[0-9]{3})" in field Variable 3, replace with "\v0$0\v0" in field Year
Find "\~" in the Year field, remove it and everything before
Find "\~" in the Year field, remove it and everything after

If there is no sequence found, the above will result in a strange Year field.

The following is safer but should be executed grouped.

Set Variable 0 to "\~"
Find first match for regular expression "(2[0-9]{3})" in field Variable 3, replace with "\v0$0\v0" in field Variable 1
Find "\~" in Variable 1, remove it and everything before, set state
if true
Find "\~" in Variable 1, remove it and everything after
Set the Year field to "\v1"
endif

Whatever works for you 🙂

I've decided to expand the functionality of the regex statement. v3.12.1 will support the current replace function and adds match extraction and match location extraction. So once, v3.12.1 is released, you can do it with a singe regex statement. 🙂