It depends on what part of that is going to be changing, but as a guess, perhaps this:
<?PHP
$str = "This is just me talking here to demonstrate this thing Trigger:[/ctcp news stuff] laaa la laa doo dii.";
preg_match_all("/Trigger:\\[(.*?)\\]/", $str, $matches);
$matches = $matches[0];
echo $matches[0]; // first match
?>