$re = '/\b(first|second|third|fourth)(?=(?:\s*(?:,|and)\s*(?:first|second|third|fourth))*\s+section\b)/';
$str = '1. something first, other and then second, third and fourth section
2. something first and other and then second, third and fourth section
3. something first, second other and then second, third and fourth section
4. fourth section
5. something first, second, third and fourth section
';
preg_match_all($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