$re = '/<{\s*_N\s*\(([^\)]*)\)\s*}>?/';
$str = '<{_N( \'some_domain_id\' , \'this can be an arbitary string\',{\'a\':\'b\',\'c\':\'d\'})}>';
preg_match_all($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