<?php
$string = 'start ** not bold ** and **bold** not bold **bold *** still bold** not bold **** not bold ****';
echo preg_replace('/(?<!\*)\*\*(?![\s*])(.*?)(?<![\s*])\*\*(?!\*)/', '<b>$1</b>', $string);
start ** not bold ** and <b>bold</b> not bold <b>bold *** still bold</b> not bold **** not bold ****
Output for 8.3.5
Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
start ** not bold ** and <b>bold</b> not bold <b>bold *** still bold</b> not bold **** not bold ****