$re = '/<span style=\"font-size:14px;\">([\w\s]+)(<\/span>)/s';
$str = '<div class="Description_Productinfo" itemprop="description"><p><span style="color:#7f8c8d;"><span style="font-family:Arial,Helvetica,sans-serif;"><span style="font-size:14px;">Alphanumeric Words If You Wish To Match</span></span></span></p></div>
';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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