- print_r: documentation ( source)
- preg_match_all: documentation ( source)
<?php
$s = "__('This is a (TEST) all of this i want') i dont want any of this; __(extract this)";
if (preg_match_all('~__(\(((?:[^()]++|(?1))*)\))~', $s, $matches)) {
print_r($matches[2]);
}