You can use two s commands
$ echo "Here is a String" | sed 's/.*Here//; s/String.*//'
is a
Also works
$ echo "Here is a StringHere is a String" | sed 's/.*Here//; s/String.*//'
is a
$ echo "Here is a StringHere is a StringHere is a StringHere is a String" | sed 's/.*Here//; s/String.*//'
is a