$re = '/https:\/\/plus\.google\.com\/u\/\d\/([^\+]\w+)/';
$str = 'https://plus.google.com/u/0/+Username
https://plus.google.com/u/2/Username/posts
https?:\\/\\/plus\\.google\\.com\\/([^\\/]*)
#хватает только "u"
https?:\\/\\/(www\\.)?instagram\\.com\\/([^\\/].+)
#instagram режет: "username/" - как убрать в конце "/" ?';
preg_match_all($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