$re = '/\bPAL\d+\b/m';
$str = 'TCNU1573105 HDPE HTA108 155 155 000893520918 PAL990 25.2750 MT 28.9750 MT';
preg_match_all($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