function for tokening a String and poll values similar to ESP8266WebServer.arg()

Projects Programming
1

hello,
I am searching for a function for tokening a String and poll values similar to ESP8266WebServer.arg()

receiving a message from a client on
ESP8266WebServer lanserver;
the lanserver gets messages from client which perhaps look like this:

&IN1=0&IN2=1&C1OUT1=10&C2OUT2=0&C1OUT3=-100

i.e.
"&" is beginning of the variable name, "=" follows the variable name, then comes the Integer value to be read,
then "&" is beginning of the next variable name

then I can retrieve the values e.g. by a ESP8266WebServer method, i.e.:

    int OUT1=0;
     String msgtok="";
     msgtok= lanserver.arg("C1OUT1");  
     if(msgtok!="") {
       OUT1=msgtok.toInt(); 
     }

Now I am receiving String messages on the
WiFiClient client;

unfortunately, opposite to ESP8266WebServer the WiFiClient client does not have a method arg().

So I retrieve the msg from the Server by

String msgline = client.readStringUntil('\r');

This msgline will look similar to the one above (but values changed arbitrarily), e.g.

&IN1=0&IN2=1&C1OUT1=0&C2OUT2=-500&C1OUT3=-100

my question:
where can I find a function which processes this entire msgline String for reading an arbitrary value related to it's specific variable name ?