$re = '/(\{\{client\.events\.(?:purchase|(add_to_cart))(?:\.\d)?\.(.*)\}\})/mi';
$str = 'ou recently ... {{client.events.add_to_cart.product_name}}, ...
..
Also
...
{{client.events.purchase.product_name}}
{{client.events.add_to_cart.product_name}}
{{client.events.add_to_cart.0.product_name}}
{{client.events.add_to_cart.1.product_name}}';
preg_match_all($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