code: private function validateHexadecimal($value)
      {
          $isValid = filter_var(
              $value,
              FILTER_VALIDATE_REGEXP,
              array(
                  "options" => array("regexp" => '`^[0-9A-Fa-f]+$`'),
              )
          );
          if ($isValid === false) {
              $this->throwInvalidArgumentMustBe("valid hexadecimal");
          }
      }
docs: Allowed characters: 0-9, a-f, A-F
  
  @param $value