<?php
$str ="test Приходите в наш <a d>test</a> видеочат, бывший когда-то test'м.testa больше нет. Забейте на <h2>test</h3> Труконф test";
echo preg_replace('/<[ah][^>]*>test\K(*SKIP)(*FAIL)|test/', '<$0>', $str).PHP_EOL;
echo preg_replace('/a{3}c/', '<$0>', 'aaaac');
Warning: preg_replace(): Compilation failed: nothing to repeat at offset 18 in /in/vEXh0 on line 4
a<aaac>
Output for 5.0.0 - 5.0.1
Warning: preg_replace(): Compilation failed: nothing to repeat at offset 18 in /in/vEXh0 on line 4
Notice: Use of undefined constant PHP_EOL - assumed 'PHP_EOL' in /in/vEXh0 on line 4
PHP_EOLa<aaac>
Output for 4.3.10
Warning: Compilation failed: nothing to repeat at offset 18 in /in/vEXh0 on line 4
a<aaac>
Output for 4.3.0 - 4.3.9
Warning: Compilation failed: nothing to repeat at offset 18 in /in/vEXh0 on line 4
Notice: Use of undefined constant PHP_EOL - assumed 'PHP_EOL' in /in/vEXh0 on line 4
PHP_EOLa<aaac>