Flexible patsubst (add something in the middle)

[Peng Yu]

I want to add something to a number of string in the middle but not at
the ends. I made the following example (doesn't work). Could you
please let me know what is the most convenient to add something to the
middle of a number strings?

.PHONY: all

PDF=uuu/vvv.pdf xxx/yyy.pdf
TMP=$(patsubst %/%.pdf, %abc/%.pdf, $(PDF))

all:
        echo $(TMP)