Thank you cripto9t!
Based on that, here is what you need to do Torborg:
In inc_func_posting look for:
fString = replace(fString, "<s>", "[s]", 1, -1, 1)
fString = replace(fString, "</s>", "[/s]", 1, -1, 1)
Below that add:
fString = replace(fString, "<sup>", "[sup]", 1, -1, 1)
fString = replace(fString, "</sup>", "[/sup]", 1, -1, 1)
fString = replace(fString, "<sub>", "[sub]", 1, -1, 1)
fString = replace(fString, "</sub>", "[/sub]", 1, -1, 1)