$re = '/^(\\\\)(\\\\[\w\.\-_]+){2,}(\\\\?)$/m';
$str = '\\\\computer-name\\drive\\optional_folder1\\optional_folder2\\
\\\\computer_name\\drive\\optional_folder1\\
\\\\computer-name\\drive\\
\\\\192.168.1.1\\drive\\
\\\\computer-name\\\\drive\\
';
preg_match_all($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