int test, test1, test2;
String valeur0;

String sensor = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?><!-- written by IQ4NC32 Iss3.51 Jun 29 2018 Saved 2023-7-10T14:43:7TZD --><TrendProject><Site CncAddress=\"101\" TuaString=\"T/IQ3 - 00:10:70:15:6E:69/TCP,,,192.168.1.100,10101///9/80/\"><Lan IsLocal=\"True\" LanNumber=\"9\"><Device DeviceNumber=\"80\" VersionString=\"IQ4NC32 Iss3.51 Jun 29 2018\"><DeviceOverview DeviceType=\"q\" MajorVersion=\"3\" MinorVersion=\"5\" HexEncoded=\"711B1B1B41333531\" Revision=\"1\"></DeviceOverview><Module Name=\"S1\" SubTypeNumber=\"-1\"><Param Name=\"$\" Type=\"2\" Value=\"BASE_AI_T-EXTERIEUR\" /><Param Name=\"V\" Type=\"1\" Value=\"19.688799\" /></Module>";
#define SEARCH_BASE_T_EXT "BASE_AI_T-EXTERIEUR"
#define SEARCH_VALUE "Value="
#define SEARCH_FIN_VALUE "\""

void setup() {
  Serial.begin(115200);
  test = sensor.indexOf(SEARCH_BASE_T_EXT);
  if(test > 0){
    test1 = sensor.indexOf(SEARCH_VALUE, test) + sizeof(SEARCH_VALUE);
    if(test1>0){
      test2 = sensor.indexOf(SEARCH_FIN_VALUE,test1);
      valeur0 = "T°EXT: " + sensor.substring(test1,test2);
      Serial.println(valeur0);
    } else {
      Serial.println("Pas de valeur trouvée pour test1");
    }
  }else{
    Serial.println("Pas de valeur trouvée pour test");
    valeur0 = "NUL";
    Serial.println(valeur0);
  }

}

void loop() {
  // put your main code here, to run repeatedly:

}