which hook do I use to create a plugin for this?

[zeroality]

I found this:

$word = array(
'google',
'yahoo'
);

$link = array(
'<a href="http://google.com">google</a>',
'<a href="http://yahoo.com">yahoo</a>'
);


$this->post['message'] = str_replace($word, $link, $this->post['message']);

from a 3.5.x modification that does exactly what I wanted to do on my forums. I wanted to replace a word with another (a bunch of them actually) in existing posts.

Just not sure which hook I should use when creating a new plugin. I've never done a custom plugin for 4.x before. Thanks!