$re = '/(?=[^\\\\]*MVP[^\\\\]*$)(?!.*Missing).*\.(?:csv|txt|xlsx?)$/m';
$str = '\\\\p8files\\MVP\\Inbound\\2023\\202310\\20231016\\BillingProvEntityFix1016.xlsx
\\\\p8files\\MVP\\Inbound\\2023\\202310\\20231016\\BillingProv_MVP_EntityFix1016.xlsx
\\\\p8files\\MVP\\Inbound\\2023\\202310\\20231016\\BillingProv_MVP_Entity_MissingFix1016.xlsx';
preg_match_all($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