$re = '/_(\w+)_\d+_\d+\.[^.]*$/m';
$str = 'Doc20191001119049_fotocontargasx_3962122_943000.jpg
Doc201810011052053_fotoAssicurazioneCartaceo_3962128_943000.jpg
Doc201910011214020_fotoesterna_ant_396024_947112.jpg
Doc201710071149010_foto_TargaMid_4007396_95010.jpg';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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