<?php

$regex = "/(?!<.*?)([\\p{Cyrillic}]+)(?![^<>]*?\\>)/umi";
$strng = "bar foo <p> barfoo </p> foobar ОртоЦентр bar bar\n";
$subst = '<a href="http://f...content-available-to-author-only...o.bar">$1</a>';
$limit = '-1';
     
$result = preg_replace($regex, $subst, $strng, $limit);

echo $result;

?>