$re = '~/\d{8}(?=/)~';
$str = '2017/07/24204301/a-4.png
2017/07/24204318/a-5-e1501986401369.png
2017/11/24211223/questao10branca-172x300.png';
$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