$re = '/sum\((?:\([^()]*\)|.)*?\)/m';
$str = 'str1 = eval(sum(feat(57),feat(57),feat(66))/feat(57));
str2 = eval(sum(feat(47),feat(55),feat(86)));
str3 = eval(feat(47)/sum(feat(51),feat(52),feat(53)));
str4 = eval(feat(63)/sum(feat(57):feat(66)));';
preg_match_all($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