What you are asking is very tricky at best. Your average Arduino doesn't have enough memory to start storing arrays of Strings.
Instead you will need to "distil" the information as it comes in and store the distilled information.
If the possible keys are all known beforehand then you can assign a number to each possible key - say RCP is 1, CMD is 2, VAL is 3, etc. If the values are all numeric or can be represented by a number, then convert those to the corresponding number and store those.
Also the Arduino is not good at working with dynamic memory allocations - again because of lack of memory. If you can decide on a reasonable maximum number of messages to store then you can pre-allocate the memory for them.
For instance it may be good to define a structure which is the content of a message:
struct message {
uint8_t key;
uint16_t val;
};
Then make an array of that structure - one slice per possible command, say 10 of them:
struct message messageQueue[10];
That would take a total of 30 bytes in memory - each entry being 3 bytes (1 for the key, two for the value). Now you can set the keys and values to the correct data as it arrives.
Depending on how you want to deal with the messages as they come in - add them to the end of an existing queue, or always starting the queue from empty every time a new message arrives - you might want to implement a circular buffer. This is basically how the Serial port on the Arduino works - in involves having two pointers to locations in the array - one where you are writing to, and one where you are reading from. As you write to the array the writing pointer moves along - if it reaches the end it starts from the beginning again. As you read from the array the reading pointer moves along too. If the two pointers are together then the array is empty. If the writing pointer would end up where the reading pointer is, the array is full. Take a look in the Arduino source code (HardwareSerial.cpp) to see how it's implemented.
As for parsing the string as it comes in - if you have it as a single String already then it's a case of looking through it for the | characters and splitting out each portion in turn, then doing the same with each portion looking for the =. Then you can interpret what is in the key/value pairs and decide what to store in the array.
