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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 - 7.1.7
[<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>]