External file links with regular expression search

The pattern I use to name my pdf files is "[Year]_[Firstauthorlastname]_[something].pdf"
To link a pdf file to a bib entry, I use the “Get fulltext” button with the regular expression search “**/.*([year]\\_[auth]).*\\.[extension]”. The results I get is the list of all files published the same year as my bib entry, meaning that the [auth] field is probably not properly recognized. Is it normal ? Would someone have a solution ? (my bibtex key pattern is “[auth3:lower][shortyear]”)