WP Plugin: Adding a custom code in editing post and rendering it

I am new into WP Plugin development. I want to make a plugin which adds the facility to use any custom code within editor to render it into something else. For instance if I am referring my Hacker News profie and I am using the code H(HackerNewsID) then after publishing it makes a URL of Hacker News pointing to my profile.

What feature of WP would be best fit for it?

Thanks