As noted, this can only be reproduced on the local URL of the forums you are using.

Bla bla http://www.phpbb.com/community/, hi there.

When doing a preview, it results in...

Bla bla http://www.phpbb.com/community,/ hi there.

The comma gets moved before the forward slash like so: ",/".

I see here in the bug tracker, I just did a preview of my post, and where I am typing now and looking at it below, I see the comma is still in the correct position, but my post being previewed above has moved the comma to the left of the forward slash.

----------

http://www.phpbb.com/community/,

... results in ...

http://www.phpbb.com/community,/

----------

Testing further... the same issue occurs with periods too I see.

http://www.phpbb.com/community/.

... results in ...

http://www.phpbb.com/community./

----------

http://www.phpbb.com/community/1

... results in ...

http://www.phpbb.com/community/1

----------

http://www.phpbb.com/community/a

... results in ...

http://www.phpbb.com/community/a