core-bundle/src/Resources/contao/library/Contao/Date.php
public static function getRegexp($strFormat=null)
	{
		if ($strFormat === null)
		{
			$strFormat = static::getNumericDateFormat();
		}

		if (!static::isNumericFormat($strFormat))
		{
			throw new \Exception(sprintf('Invalid date format "%s"', $strFormat));
		}

		return preg_replace_callback('/[a-zA-Z]/', function ($matches)
			{
				// Thanks to Christian Labuda
				$arrRegexp = array
				(
					'a' => '(?P<a>am|pm)',
					'A' => '(?P<A>AM|PM)',
					'd' => '(?P<d>0[1-9]|[12][0-9]|3[01])',
					'g' => '(?P<g>[1-9]|1[0-2])',
					'G' => '(?P<G>[0-9]|1[0-9]|2[0-3])',
					'h' => '(?P<h>0[1-9]|1[0-2])',
					'H' => '(?P<H>[01][0-9]|2[0-3])',
					'i' => '(?P<i>[0-5][0-9])',
					'j' => '(?P<j>[1-9]|[12][0-9]|3[01])',
					'm' => '(?P<m>0[1-9]|1[0-2])',
					'n' => '(?P<n>[1-9]|1[0-2])',
					's' => '(?P<s>[0-5][0-9])',
					'Y' => '(?P<Y>[0-9]{4})',
					'y' => '(?P<y>[0-9]{2})',
				);

				return $arrRegexp[$matches[0]] ?? $matches[0];
			}, preg_quote($strFormat));
	}
public static function getRegexp($strFormat=null)
	{
		if ($strFormat === null)
		{
			$strFormat = static::getNumericDateFormat();
		}

		if (!static::isNumericFormat($strFormat))
		{
			throw new \Exception(sprintf('Invalid date format "%s"', $strFormat));
		}

		return preg_replace_callback('/[a-zA-Z]/', function ($matches)
			{
				// Thanks to Christian Labuda
				$arrRegexp = array
				(
					'a' => '(?P<a>am|pm)',
					'A' => '(?P<A>AM|PM)',
					'd' => '(?P<d>0[1-9]|[12][0-9]|3[01])',
					'g' => '(?P<g>[1-9]|1[0-2])',
					'G' => '(?P<G>[0-9]|1[0-9]|2[0-3])',
					'h' => '(?P<h>0[1-9]|1[0-2])',
					'H' => '(?P<H>[01][0-9]|2[0-3])',
					'i' => '(?P<i>[0-5][0-9])',
					'j' => '(?P<j>[1-9]|[12][0-9]|3[01])',
					'm' => '(?P<m>0[1-9]|1[0-2])',
					'n' => '(?P<n>[1-9]|1[0-2])',
					's' => '(?P<s>[0-5][0-9])',
					'Y' => '(?P<Y>[0-9]{4})',
					'y' => '(?P<y>[0-9]{2})',
				);

				return $arrRegexp[$matches[0]] ?? $matches[0];
			}, preg_quote($strFormat));
	}
Return a regular expression to check a date

@param string $strFormat An optional format string

@return string The regular expression string

@throws \Exception If $strFormat is invalid