<?php

$text    = '
first line without links
<a title="bla-bla" href="http://content-available-to-author-only.ru/">second string linelink </a>
third line';


$pattern = '/^(?>(?!(\<a.*href)|\<\/a).)*$/umi';
$result  = preg_match_all($pattern, $text, $matches);
print_r($matches);