$re = '/(?<=Amount Due|Pay This Amount|Total Balance|Total Current Month’s Charges|Amount Now Due|Amount Due|Amount due on|Total Balance on).*\$([\d.,]+)$/mi';
$str = 'Pay this amount $45.50
Pay this amount due Mar04 $56.67
Amount due $56.78
Amount due on dec4 $76.45
Total balance $76.45
Total balance on apr3 $64.34';
preg_match_all($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