You must be using a server-side on the backend to post the comments. Which PHP a simple str_replace, preg_replace would work. The link is a little (actually, alot) more complicated. It's skimming the surface of BBCode.
Here's the answer for the line breaks:
Code:
<?php
$new = str_replace("\n","<br>",$originaldata)
//do something with $new (echo for example)
?>
Whereas you would change $originaldata to what was being sent.
It would help more if you described the situation better, or gave the code.
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