f() { for i; do echo "|$i|"; done; }
v=xx
f ${v/'x'/"y z"}
f ${v/"x"/'y z'}
f "${v/'x'/"y z"}"
f "${v/"x"/'y z'}"
f ${v//'x'/"y z"}
f ${v//"x"/'y z'}
f "${v//'x'/"y z"}"
f "${v//"x"/'y z'}"