$re = '/(?<=last_name\\\\\":\\\\\")(\w)+(?=\\\\)/m';
$str = '{"message":"{\\"_\\":\\"user\\",\\"pFlags\\":{\\"contact\\":true},\\"user_flags\\":2143,\\"id\\":702212125,\\"access_hash\\":\\"914250561826\\",\\"first_name\\":\\"david\\",\\"last_name\\":\\"jones\\",\\"username\\":\\"david_d192\\",\\"phone\\":\\"051863329875\\",\\"status\\":{\\"_\\":\\"userStatusRecently\\"}}","phone":"051863329875","version":"3","type":"unknown","token":"1556189892619764206","p_id":702212125,"username":"david_d192","type":"redis","user_flags":2143,"host":"win",from":"contacts"}
{"index": {"_type": "_doc", "_id": "36GG54F"}}
{"message":"{\\"_\\":\\"user\\",\\"pFlags\\":{\\"contact\\":true},\\"user_flags\\":2143,\\"id\\":702212125,\\"access_hash\\":\\"914250561826\\",\\"first_name\\":\\"david\\",\\"last_name\\":\\"jones\\",\\"username\\":\\"david_d192\\",\\"phone\\":\\"051863329875\\",\\"status\\":{\\"_\\":\\"userStatusRecently\\"}}","phone":"051863329875","version":"3","type":"unknown","token":"1556189892619764206","p_id":702212125,"username":"david_d192","type":"redis","user_flags":2143,"host":"win",from":"contacts"}
{"index": {"_type": "_doc", "_id": "36GG54F"}}
{"message":"{\\"_\\":\\"user\\",\\"pFlags\\":{\\"contact\\":true},\\"user_flags\\":2143,\\"id\\":702212125,\\"access_hash\\":\\"914250561826\\",\\"first_name\\":\\"david\\",\\"last_name\\":\\"jones\\",\\"phone\\":\\"051863329875\\",\\"status\\":{\\"_\\":\\"userStatusRecently\\"}}","phone":"051863329875","version":"3","type":"unknown","token":"1556189892619764206","p_id":702212125,"type":"redis","user_flags":2143,"host":"win",from":"contacts"}
{"index": {"_type": "_doc", "_id": "36GG54F"}}
#Target: id, first_name , last_name , phone , username(if exist)';
preg_match_all($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