/html

[MangaD]

Did you consider a normal string-manipulation approach? That might be simpler to get right than getting regular expressions to work.

I think maybe writing a plugin (php) for tables bbcode might work. But I haven't had the time to take a look at how MyBB plugins work exactly, I'd like to hear BP's opinion on this.

Edit: What are some examples that may cause a problem in the layout

Causing problems in the layout is very easy, example would be using <td> or <tr> outside a <table>, the results would look like this:

[tr][td]some text[/td][/tr]: http://s6.postimg.org/cl5u9r8rl/tr_td.png
[td]some text[/td]: http://s6.postimg.org/e1hcrwboh/image.png

This is actually possible to do at HFE right now, but I don't expect people to do it of course.

or become a security exploit?

I'm actually curious about this too. As far as I know, scripts are not possible to use in MyBB. Iframes are possible (and therefore the scripts inside them), but shouldn't be hard to disable them. @Someone else explained it here. But somehow his argument didn't convince me. However, just to play it safe, I have to agree with the decision to disable html, less chances of something bad to happen.