osx - Why these simple shell commands fail when used in sed's replacement part -

while trying find answer of this sed question came strange behavior couldn't understand.

let's have file called data

$> cat data foo.png abcd.png bar.png baz.png

the task use sed in line replace lines uppercase ascii characters lowercase. output should be:

$> cat data foo.png abcd.png bar.png baz.png

the solution should work on non-gnu sed sed on mac

i attempted embedded awk sed's replacement part:

sed -e 's/[^ ]*[a-z][^ ]*.png/'$(echo \&|awk '{printf("<%s>[%s]",$0, tolower($0))}')'/' data

strangely outputs this:

foo.png <abcd.png>[abcd.png] bar.png <baz.png>[baz.png]

as can see sed picking right lines uppercase alphabets, , that's reaching awk tolower() function of awk failing , producing same text input.

can shell expert please explain weird behavior.

sed 'y/abcdefghijklmnopqrsyuvwxyz/abcdefghijklmnopqrstuvwxyz/'

KHS