$re = '/^(?<AC>\d{3})(?<First>\d{3})(?<Last>\d{4})\s(?<EXT>(E?X?T?e?x?t?\.?\s?\d{1,7})?)/m';
$str = '5133521652 extension 1234
5133521652 Extension 1234
5133521652 extension 1234
5133521652 e 1234
5133521652 x 1234
5133521652 t 1234
5133521652 ex 1234
5133521652 ext 1234
5133521652 E 1234
5133521652 X 1234
5133521652 T 1234
5133521652 e. 1234
5133521652 X 1234
5133521652 Ext 1234
5133521652 EXT 1234
5133521652 EXT. 1234
';
preg_match_all($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