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char myChar = '3'; // try with '#' for a non alphanumeric char 

void setup() {
  Serial.begin(9600);

  if (isAlphaNumeric(myChar)) {  // tests if myChar is a letter or a number
    Serial.println("The character is alphanumeric");
  } else {
    Serial.println("The character is not alphanumeric");
  }
}

void loop() {
}