(xref http://cairnarvon.rotahall.org/2010/05/25/towards-a-better-bbcode/)
(xref http://cairnarvon.rotahall.org/misc/sexpcode.html)

{b This} is {i {u export} {o mark-up}}

<b>This</b> is <i><u>export</u> <o>mark-up</o></i>

{b.i.o.u EXPERT}

<b><i><o><u>EXPERT</u></o></i></b>

{sup*2 This text is superscripted two levels}

<sup><sup>This text is superscripted two levels</sup></sup>

n{sup 2}

n^2

{b.sup*2.i Way to be a dick, {sub*2.u.o dick}.}

<b><sup><sup><i>Way to be a dick, 
  <sub><sub><u><o>dick</o></u></sub></sub></i></sup></sup></b>

.