$re = '/Source:\s*\K.+\b/m';
$str = 'Art 1 This is article one. Source: The Guardian.
Art 2 This is article two. Source: New York Times.
Art 3 This is article three. Source: The Washington Post.
';
preg_match_all($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