Search for a certain combination of strings in serial and read the following characters until another set character stops the search

Question

I have the following serial message that I'm receiving from an ESP32 running GRBL:

<Idle|MPos:158.500,0.000,0.000|FS:0,0|Pn:H|Ov:100,100,100|SD:0.21,/instruction.nc>

I want to check the value after SD: which in this case would be 0.21. It is a value that represents the percentage of a file being read, so values between 100.00 and 0.00 are being sent.

My guess is to look for the SD: in the string and then read all characters until the , but I can't really find a way to realize that.

I found the startsWith() and the substring()function that are able to look for a set of characters in a string, but it seems like they can only check a set amount of characters which in this case doesn't work as 0.00 is 4 characters and 100.00 is 6 characters.

Any help is much appreciated!!

• Majenko: I'd split the string on | then for each substring split it on :. You then have key/value pairs that are easy to compare and work with. Use a C string not a String object and strtok_r and strcmp to do the job with minimal memory outlay. I'll write a proper answer later, I'm on my phone at the moment.
• heljo: @Majenko thanks for the super quick reply! I 'll give it a try as far as I can!

Answer 1

Firstly you should really be working with a C string (char *) instead of a String object, otherwise you will be massively fragmenting your heap with the string manipulations to come.

Once you have your data in a C string (char array) you can use strtok_r to split the string on the | character, then for each substring further split it on the :. You need the _r variant of strtok to make it "re-entrant" so you can have two tokenizations at once nested.

For example (untested):

char *data = "<Idle|MPos:158.500,0.000,0.000|FS:0,0|Pn:H|Ov:100,100,100|SD:0.21,/instruction.nc>";
char *pair; // Pointer to a K/V pair
char *pairptr = data; // Pointer for outer strtok
// Split the string on the first |
pair = strtok_r(data, "|", &pairptr);
// While we have a K/V pair
while (pair) {
 char *kvptr = pair;
 // Split the substring into two parts on the colon
 char *key = strtok_r(pair, ":", &kvptr);
 char *val = strtok_r(NULL, ":", &kvptr);
 if (val) { // If we split the data correctly:
 if (strcmp(key, "SD") == 0) { // If the key is "SD" then
 Serial.print("SD data: ");
 Serial.println(val);
 }
 }
 // Get the next string portion
 pair = strtok_r(NULL, "|", &pairptr);
}

Once you have your val you can further split that down on commas in the same way as it's been split on the colon.