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<?php $content = '[btn href="(.?)" "(.?)"]'; echo preg_replace('#[btn href="(.*?)" "(.*?)"]#i', '<a href="$1" class="btn"> $2</a>', $content);
based on KUi3q
Output for 4.3.0 - 5.6.28, hhvm-3.12.0, 7.0.0 - 7.1.0
[<a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a><a href="" class="btn"> </a>]
Output for hhvm-3.10.0

Process exited with code 153.