You are trying to perform an indirect assignment.
You should get rid of these two lines :
index=\$$index
echo $index;
By simply writing :
echo "${!index}"
Which does an indirect expansion cleanly (gives you the value of the variable whose name is contained in variable index).
Next, the problematic line is this one:
$index="$(grep -oE '\$${!index} = .*;' ... (rest omitted)
In Bash, an assignment cannot begin with a $.
One way you can perform an indirect assignment is this (after removing the index re-assignment as suggested above) :
printf -v "$index" "$(grep -oE '\$${!index} = .*;' ... (rest omitted)
This uses the -v option of printf, which causes the value passed as the final argument to be assigned to a variable, the name of which is passed to the -v option.
There are other ways of handling indirect assignment/expansions, some with code injection risks, as they use (potentially uncontrolled) data as code. This is something you may want to research further.
Please note I am assuming the actual grep command substitution works (I have not tested).
