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Here is Arduino method to split a String as answer to the question "How to split a string in substring?" declared as a duplicate of the present question.

The objective of the solution is to parse a series of GPS positions logged into a SD card file. Instead of having a String received from Serial, the String is read from file.

The function is StringSplit() parse a String sLine = "1.12345,4.56789,hello" to 3 Strings sParams[0]="1.12345", sParams[1]="4.56789" & sParams[2]="hello".

1. String sInput: the input lines to be parsed,
2. char cDelim: the delimiter character between parameters,
3. String sParams[]: the output array of parameters,
4. int iMaxParams: the maximum number of parameters,
5. Output int: the number of parsed parameters,

The function is based on String::indexOf() and String::substring() :

int StringSplit(String sInput, char cDelim, String sParams[], int iMaxParams)
{
 int iParamCount = 0;
 int iPosDelim, iPosStart = 0;
 do {
 // Searching the delimiter using indexOf()
 iPosDelim = sInput.indexOf(cDelim,iPosStart);
 if (iPosDelim > (iPosStart+1)) {
 // Adding a new parameter using substring() 
 sParams[iParamCount] = sInput.substring(iPosStart,iPosDelim-1);
 iParamCount++;
 // Checking the number of parameters
 if (iParamCount >= iMaxParams) {
 return (iParamCount);
 }
 iPosStart = iPosDelim + 1;
 }
 } while (iPosDelim >= 0);
 if (iParamCount < iMaxParams) {
 // Adding the last parameter as the end of the line
 sParams[iParamCount] = sInput.substring(iPosStart);
 iParamCount++;
 }
 return (iParamCount);
}

And the usage is really simple:

String sParams[3];
int iCount, i;
String sLine;
// reading the line from file
sLine = readLine();
// parse only if exists
if (sLine.length() > 0) {
 // parse the line
 iCount = StringSplit(sLine,',',sParams,3);
 // print the extracted paramters
 for(i=0;i<iCount;i++) {
 Serial.print(sParams[i]);
 }
 Serial.println("");
}