hook for posting data?

[orange gold]

I have coded alot in my days but I am new to VB and PHP.. Anyways I want a simple peice of code that will go through a users post right before they post it or preview it and replace string 'x' with string 'y'..

Lets say for the sake of exmaple it will detect the word "infinity" and replace it with the character "∞"

This is what I have so far (for the main part of the plugin) and I have no clue where to go? any guiding hands??

<plugin active="1" executionorder="5">
			<title>Math</title>
			<hookname>newpost_process</hookname>
			<phpcode><![CDATA[if (THIS_SCRIPT == "index" and $vbulletin->options['math_onoff']) {
                $mathpost .= '';
                $template_hook['postbit_messagearea_start'] .= 'mathpost'; 
}]]></phpcode>
		</plugin>