Modify before save using plugin?

[freeones_raymon]

Hello,

I want to modify new posts, edited posts or new threads before they are saved into the database, by adding a special tag with information around specified words. But, I do not want the users to see those tags, because they could be "abused" then. Before displaying I convert those tags into links.

So when someone edits a post I have to remove those tags and when someone saves it , those tags have to be added again. But when the user gets an error like (message too large, etc), the tags must also not be visible!

So I need to modify it at the latest point possible. Is there anyone who can tell me which hooks and variables to use for this, because I cannot seem to find it...

Thanks in advance,
Raymond