String input = "0x12";

int output = 0;

void setup() {
  Serial.begin(9600);
  output = hexStringToDecimal(input);
  Serial.println(output);

}

void loop() {
  // put your main code here, to run repeatedly:

}

long hexStringToDecimal(String hexString) {
  long decimalValue = 0;
  int hexLength = hexString.length();
  
  for (int i = 0; i < hexLength; i++) {
    char c = hexString.charAt(i);
    int digitValue;
    
    if (c >= '0' && c <= '9') {
      digitValue = c - '0';
    } else if (c >= 'A' && c <= 'F') {
      digitValue = 10 + (c - 'A');
    } else if (c >= 'a' && c <= 'f') {
      digitValue = 10 + (c - 'a');
    } else {
      // Handle invalid characters if necessary
      continue;
    }
    
    decimalValue = (decimalValue * 16) + digitValue;
  }
  
  return decimalValue;
}