Contao\Date::getInputFormat PHP Method

    public static function getInputFormat($strFormat = null)
    {
        if ($strFormat === null) {
            $strFormat = static::getNumericDateFormat();
        }
        if (!static::isNumericFormat($strFormat)) {
            throw new \Exception(sprintf('Invalid date format "%s"', $strFormat));
        }
        $arrCharacterMapper = array('a' => 'am', 'A' => 'AM', 'd' => 'DD', 'j' => 'D', 'm' => 'MM', 'n' => 'M', 'y' => 'YY', 'Y' => 'YYYY', 'h' => 'hh', 'H' => 'hh', 'g' => 'h', 'G' => 'h', 'i' => 'mm', 's' => 'ss');
        $arrInputFormat = array();
        $arrCharacters = str_split($strFormat);
        foreach ($arrCharacters as $strCharacter) {
            if (isset($arrCharacterMapper[$strCharacter])) {
                $arrInputFormat[$strFormat] .= $arrCharacterMapper[$strCharacter];
            } else {
                $arrInputFormat[$strFormat] .= $strCharacter;
            }
        }
        return $arrInputFormat[$strFormat];
    }

 /**
  * Process a custom date regexp on a widget.
  *
  * @param string $rgxp   The rgxp being evaluated.
  *
  * @param string $value  The value to check.
  *
  * @param Widget $widget The widget to process.
  *
  * @return void
  *
  * @SuppressWarnings(PHPMD.Superglobals)
  * @SuppressWarnings(PHPMD.CamelCaseVariableName)
  */
 public static function processCustomDateRegexp($rgxp, $value, $widget)
 {
     if ('MetaModelsFilterRangeDateRgXp' !== $rgxp) {
         return;
     }
     $format = $widget->dateformat;
     if (!preg_match('~^' . Date::getRegexp($format) . '$~i', $value)) {
         $widget->addError(sprintf($GLOBALS['TL_LANG']['ERR']['date'], Date::getInputFormat($format)));
     } else {
         // Validate the date (see https://github.com/contao/core/issues/5086)
         try {
             new Date($value, $format);
         } catch (\OutOfBoundsException $e) {
             $widget->addError(sprintf($GLOBALS['TL_LANG']['ERR']['invalidDate'], $value));
         }
     }
 }