Example of looking for specific text from serial input stream

strstrSerial.ino

char buffer[5];
#define buffer_size sizeof(buffer)/sizeof(buffer[0])
int8_t buffer_pos = 0;

void setup() {
  //Initialize serial and wait for port to open:
  Serial.begin(115200);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }
  Serial.println("Serial to mySerial demo started");
  Serial.print("buffer_size = ");  Serial.print(buffer_size); Serial.println("");

  buffer_pos = 0;
}

void loop() {
  char inByte;
  while (Serial.available()) {
    inByte = Serial.read();
    Serial.print("inByte = ");  Serial.print(inByte); Serial.println("");
    Serial.print("buffer_pos = ");  Serial.print(buffer_pos); Serial.println("");

    while (buffer_pos >= buffer_size) {
      for (int8_t i = 0; i < buffer_size - 1; i++) { // rotate buffer left if at end of buffer
        buffer[i] = buffer[i + 1];
      }
      buffer_pos--;
    }
    buffer[buffer_pos++] = inByte;
    Serial.print("buffer = ");  
    for (int8_t i = 0; i < buffer_size; i++) {
      Serial.print(buffer[i]);
    }
    Serial.println("");

    if (strcasestr(buffer, "abc")) {
      Serial.println("BINGO");
    } else {
      Serial.println("No Joy");
    }
  }

}

Here is example out. Note that inByte is the character sent from a serial stream. The trigger text was ABC either upper or lower case.

Serial to mySerial demo started
buffer_size = 5
inByte = 1
buffer_pos = 0
buffer = 1
No Joy
inByte = a
buffer_pos = 1
buffer = 1a
No Joy
inByte = b
buffer_pos = 2
buffer = 1ab
No Joy
inByte = c
buffer_pos = 3
buffer = 1abc
BINGO
inByte = d
buffer_pos = 4
buffer = 1abcd
BINGO
inByte = e
buffer_pos = 5
buffer = abcde
BINGO
inByte = f
buffer_pos = 5
buffer = bcdef
No Joy
inByte = 1
buffer_pos = 5
buffer = cdef1
No Joy
inByte = 2
buffer_pos = 5
buffer = def12
No Joy
inByte = 3
buffer_pos = 5
buffer = ef123
No Joy
inByte = a
buffer_pos = 5
buffer = f123a
No Joy
inByte = b
buffer_pos = 5
buffer = 123ab
No Joy
inByte = c
buffer_pos = 5
buffer = 23abc
BINGO
inByte = 1
buffer_pos = 5
buffer = 3abc1
BINGO
inByte = 2
buffer_pos = 5
buffer = abc12
BINGO
inByte = 3
buffer_pos = 5
buffer = bc123
No Joy
inByte = A
buffer_pos = 5
buffer = c123A
No Joy
inByte = B
buffer_pos = 5
buffer = 123AB
No Joy
inByte = C
buffer_pos = 5
buffer = 23ABC
BINGO
inByte = 1
buffer_pos = 5
buffer = 3ABC1
BINGO
inByte = 2
buffer_pos = 5
buffer = ABC12
BINGO
inByte = 3
buffer_pos = 5
buffer = BC123
No Joy