Get only part of file name and delete from server folder in asp.net vb

I’m trying get get only the last part of a file name in a server folder between the “_” and “.pdf” in order to delete specific files only.
The files in the invoices folder are like this: Invoice_12345.pdf, Invoice_2345.pdf, Invoice_5555.pdf, etc. I need to extract the “12345”, “2345”, and “5555”, then I need to delete the file if it is not contained in a database query.