If IN_STRING = 0 Then
  If RX Byte = ESC Then
    IN_STRING = 1
    BUF_INDEX = 0
  End If
Else
  STORE RX BYTE TO BUF @ BUF_INDEX
  BUF_INDEX = BUF_INDEX + 1
  If RX Byte = CR Then
    ....you have your string in buffer so 
        do what you need to do with it
    IN_STRING = 0  get ready for next string.
  End If
End If