3v4l.org

run code in 300+ PHP versions simultaneously
<?php $string = 'Medium 4Y3FT129# Large 4Y3FT119#'; var_dump(preg_replace('~(([A-Z0-9])*#)~', ' #', $string));
Output for 7.1.0 - 7.1.20, 7.2.0 - 7.2.33, 7.3.16 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
string(18) "Medium # Large #"

preferences:
145.97 ms | 403 KiB | 159 Q