serial communication problem

Projects Programming
12

I did it like this:

       // turn off the LED:
   if (readString.startsWith("off", 2)) {
      digitalWrite(ledPin, LOW);
    }

It works fine, but i was looking for a "if string contains" function.
Ive hooked it up wireless now and there is lots of junk in the air, and the index 2 wont make sense anymore now.