NGINX - OpenResty - How to reverse proxy a call to 2 different servers based based on a string?

I'm trying to configure NGINX/OpenResty to proxy SOAP calls to 2 different servers based on a string present on the SOAP request.

What I'm able to do:
I am able to proxy requests to 2 different servers based on the path the SOAP client is calling:
location /pathA {
proxy_pass http://www.ServerA.com/PathA/;
}
location /pathB {
proxy_pass http://www.ServerB.com/PathB/;
}

What I can't do:
I can't separate the traffic based on the content of the request. The main reason I believe is that I can't correctly assemble the LUA script to extract the information and later use it to proxy the request.

location / {
conten_by_lua '
ngx.req.read_body()
local match = ngx.re.match(ngx.var.request_body,"STRING TO FIND")
if match then
proxy_pass http://www.ServerA.com/PathA/;
else
proxy_pass http://www.ServerB.com/PathB/;

how can I achieve this?

I installed OpenResty and LUA is working fine.

I think I read somewhere that if the request is not an HTTP POST "ngx.req.read_body()" would not work. Is that correct?

Thank you for your help.