Extracting structured data from file names

[SF Markus Elfring]

Hello,

I can generate a few file names with an approach
like the following.

MY_FILES::=
$(foreach index,$(shell seq 2 -1 0),$(eval MY_FILES+=X$(index).txt))

all: $(MY_FILES)
        @echo '|$^|'

$(MY_FILES): ;


Now I am looking for a way to extract the number from
the shown prerequisite.
Can I get such encoded data by standard make and
shell functions?

Regards,
Markus