html to bbcode addon in making..

[Michael.A]

most of my member they use html is there post (( <a href= , </a> ))
so i decided to make an add-on for this but i real dont know where shod i be looking

i need to know what hook name do i use?
and what php code do i run in post before its saves the post?

PHP Code:

function make_bbcode($normal)
{
$samurl=str_replace(&#8220;<a href=”, “[url=", $normal);
$samurl=str_replace("</a>", "[/url]“, $samurl);
$samurl=str_replace(“>”, “]”, $samurl);
return($samurl);
}