$re = '/TXN[a-zA-Z0-9]*/m';
$str = 'TXNIDVLDYLGREJF2JI
TXNIDVLDJFRHOUPBVI
TXNIDVLDNEDQC2WUAH
TXNIDVLDYEATXQ5GG6
TXNIDVLDFIGZO5YUNT
TXNIDVLDFIGZ05YUNT
TXNIDVLDIPTI04G8UV
TXNIDVLD08ZFNV4CMY
TXNVIBHCLMS572889
';
preg_match_all($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