$re = '/[^\\\\]*(?=\\\\[A-Z0-9]{40}$)/m';
$str = '[hello]
PSPath
Microsoft.PowerShell.Security\\Certificate::CurrentUser
Microsoft.PowerShell.Security\\Certificate::SmartCardRoot
Microsoft.PowerShell.Security\\Certificate::Root
Microsoft.PowerShell.Security\\Certificate::CurrentUser\\Root\\CDD4EEAE6000AC7F40C3802C171E30148030C072
Microsoft.PowerShell.Security\\Certificate::CurrentUser\\Root\\BE36A4562FB2EE05DBB3D32323ADF445084ED656
Microsoft.PowerShell.Security\\Certificate::CurrentUser\\Root\\A43489159A520F0D93D032CCAF37E7FE20A8B419
Microsoft.PowerShell.Security\\Certificate::CurrentUser\\Root\\92B46C76E13054E104F230517E6E504D43AB10B5
Microsoft.PowerShell.Security\\Certificate::CurrentUser\\Root\\8F43288AD272F3103B6FB1428485EA3014C0BCFE
';
preg_match_all($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