How to hook into wordpress user comments (for parsing)

I wrote my own template, and I use a comments.php file. At the end of the file it calls comments_template() to display the mostly default wordpress comments.

I want to give users the ability to use a custom tag, similar to a bbcode (ex: [quote]blah blah[/quote] ), but rather than just have basic style properties, I need to perform some operations on the characters within the code beyond just simple styling.

I am fully capable of parsing the text. But I can't figure out how to get the user comments to parse for the tags/code in the first place. How can I hook into the comment posting function, so that I can edit and return a new string?