$re = '/^(?<tgl>\d{2})\/(?<bln>[0-1][0-9]{1,2})\/(?<thn>\d{4})$/s';
$str = '90/09/2018';
$subst = "Gafur";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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