3v4l.org

run code in 500+ PHP versions simultaneously
<?php $str = '<p>"bla" bla</p>'; echo preg_replace( '~<p>\W*\K\w~', '<span class="first_letter">$0</span>', $str );
Output for 8.0.1 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.30, 8.3.0 - 8.3.30, 8.4.1 - 8.4.18, 8.5.0 - 8.5.3
<p>"<span class="first_letter">b</span>la" bla</p>

preferences:
83.2 ms | 1105 KiB | 4 Q