easy hello world [tag] plugin?

Hi everyone, l’m looking to create a couple custom “bbcode” like plugins and was wondering if someone could help me out w/it.

What l need: Say l put [hi] inside of one of my posts, it would be replaced with “hello world”, in the actual post that users see. No variables or anything need to be sent just an easy replacement.

Thanks to anyone who can lend a helping hand!