I want to extract the year (e.g. 1967) from a text file of song information (a single line per mp3). A solution I finally came up with was to import the line into the year tag location and then apply a tag to tag conversion using $regexp(%year%,'^.(\d\d\d\d).*$',$1). But I could not find a way to do it directly. I was looking for something like, $regexp(%line_from_file%,'^.(\d\d\d\d).*$',%year%) but I could not find an appropriate tag or mapping to reference the text file line.
It is not possible to mix functions of a generating format string with the field assignment in a matching format string.
The way to import the data "as is" first into a field and then treat it is the only way.