String txtMsg = "";                         // a string for incoming text
unsigned int lastStringLength = txtMsg.length();     // previous length of the String

void setup() {

  // Open serial communications and wait for port to open:

  Serial.begin(9600);

  while (!Serial) {

    ; // wait for serial port to connect. Needed for native USB port only

  }

  // send an intro:

  Serial.println("\n\nString  length():");

  Serial.println();
}

void loop() {

  // add any incoming characters to the String:

  while (Serial.available() > 0) {

    char inChar = Serial.read();

    txtMsg += inChar;

  }

  // print the message and a notice if it's changed:

  if (txtMsg.length() != lastStringLength) {

    Serial.println(txtMsg);

    Serial.println(txtMsg.length());

    // if the String's longer than 140 characters, complain:

    if (txtMsg.length() < 10) {

      Serial.println("That's a perfectly acceptable text message");

    } else {

      Serial.println("That's too long for a text message.");

    }

    // note the length for next time through the loop:

    lastStringLength = txtMsg.length();

  }
}