FWIW, the following watch file works around this (the versioned file to be found inside a versioned directory):
version=3 http://sourceforge.net/projects/dia-installer/files/dia-shapes/(\d+\.\d+\.\d+)/ dia-shapes-(.+)\.tar\.xz/download -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]

