$re = '~ (\h*\( # capture groups, open brackets
[^)]*? # match everything BUT a closing bracket lazily
(?i:picture|see|lorem) # up to one of the words, case insensitive
[^)]*? # same construct as above
\)) # up to a closing bracket~x';
$str = 'Hello world (see below).
Lorem ipsum (there is a picture here) world!
Attack on titan (is lorem) great but (should not be removed).';
preg_match_all($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