$re = '/@\[(.*?)\]\(contact:(\d+)\)/';
$str = 'This is a string which has a @[User Full Name](contact:1) data inside.
This is a string which has a @[User Full Name](contact:1) data inside. and it can also contain many other users data like @[Second Username](contact:2) and @[Third username](contact:3) and so many others....';
preg_match_all($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