$re = '/(?(DEFINE)
(?<lowercase>(?=[^a-z]*[a-z])) # lowercase
(?<uppercase>(?=[^A-Z]*[A-Z])) # uppercase
(?<anA>A(?=B))
)
^(?&lowercase)(.*?)((?&anA)).*/mx';
$str = 'this is an A
A
ABC
';
preg_match_all($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