$re = '/[a-zA-Z\d\s]+(?:_|\s)(\d{1,2}\.\d{1,2}\.\d{2,4})/';
$str = 'AC SCR063_6.8.15.xlsx
AC SCR064_6.22.15_REVISED.xlsx
AccentCare July 2015 Rent Report 06.26.15 Final.xlsx
AccentCare June 2015 Rent Report 05.26.15 Final.xlsx';
preg_match_all($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