$re = '/(?<=H\d{3}\b)(?:(?!\+?H\d{3}\b)[\S\s])+(?=[\s\S]*\r?\nEnd word\b)/m';
$str = 'Sample text
Sample text
Sample text
H319 asdkjczixuqweoiurqoiweqwrjasdkjfqwe qweiouqwroiu kjasdkj czkjxklqjwekjiouasdiiaosudou
oiuasodiucxzlkjqweoiu oqiwur H320 asdkqjwe askjdq xzc
H325 asjdhasjd zxcjh
H331+H341+H341 askdjvkjzx qweqrqwoe
End word
Sample text
Sample text';
preg_match_all($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