ATMega 328p: Read characters after keyword

substring.ino

void setup() {
  
  Serial.begin(9600);

    String stringValue;
    String stringValue1;
    String stringValue2;

    stringValue = "ReadData: 1997";

    int index = stringValue.indexOf(':'); //keyword
    stringValue2 = stringValue.substring(index + 2);
    Serial.println(stringValue2); //1997
}

void loop() {

}