I used an mod for SEF URLs in PunBB 1.2.x. The URLs looked like:
for post: pxxx.html (p1938.html)
for topic: txxx.html (t282.html)
for forum: fxxx.html (f2.html).
However, the SEF URL scheme is different in 1.3.x. As a result, when I enable SEF URL in new punbb old SEF URLs don't work anymore. That's why I could not use new SEF scheme in PunBB 1.3.x. However, today I planned to make a solution.
I have enabled File based URL Scheme. then I have appended the following lines in include/url/File_based/rewrite_rules.php
'/^t[\/_-]?([0-9]+)(\.html?|\/)?$/i' => 'viewtopic.php?id=$1',
'/^p[\/_-]?([0-9]+)(\.html?|\/)?$/i' => 'viewtopic.php?pid=$1',
'/^f[\/_-]?([0-9]+)(\.html?|\/)?$/i' => 'viewforum.php?id=$1',
i can't explain these as I am poor at Regex. However, it apparently worked for me. I will test it for a few days though .