Should be replaced (using $prefix here, but equally viable for $exec_prefix). Each quoted line is a path using a raw string.

r"$prefix"
r"$prefix/foo"

and on Windows:

r"$prefix\bar"

These should *not* be replaced:

r"$prefixvar/subdir"
r"not/a/$prefix"
r"$prefix spacevar/subdir"

because here, the `$prefix` is part of another path component. This does leave the directory literally named "$prefix" in a bit of a pickle.

Like I said, it's niche, but if we're going to fix it, let's at least consider handling more cases properly.