$re = '/(?<=a2=)[^;]*(?:;|$)/m';
$str = 'a2=new something
a1=asdasdsad;a2=old something;a3=asadasdsadsa
a2=Some place;alksndklsand;a1=asdklsad';
preg_match_all($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