$re = '/^(1|2)\d{3}\-(0[1-9]|1[0-2])\-((0[1-9])|((1|2)\d)|3(0|1))(T|\s)((0|1)\d|2[0-3]):([0-5]\d):([0-5]\d)((\+0\d|\+1[0-4]|-0\d|-1[0-2])([0-5]d)?)?|(\d{10-13})/';
$str = '2018-05-25 23:59:59';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php