GREP for endnote reference chains
I am processing a textbook that has a formatting situation I've never had to deal with in bulk — multiple endnote references in a row. I've encountered books with the occasional twofer, and those are easy enough to spot and adjust during normal formatting, but this book has a dozen or more per chapter, and for all reasons of efficiency and consistency I'd like to automate the process.
Here's a typical clip—

The text style is BODY and may be one of several variants.
The endnote style REFS is applied by the Endnote setup of the document, which puts all elements in the style (i.e., leaves the reference marker Normal, not superscripted). It can be assumed that there will be two to five markers in a row, with a comma but no spaces between them. (There should be NO spaces ahead of markers, and one ONLY after the last.)
(Poorly chosen example, perhaps, in that the comma ahead of the first marker is common and needs to remain as is, but most markers will follow word text or normal punctuation. Also, the red highlight is just a formatting convenience that will be changed to black or 80%K in the final.)
All I want to do is apply REFS to those commas. and I've found partial solutions, but neither lookahead nor lookbehind seems to be able to handle the ~U marker wildcard.
