the text

c < d

causes a <d> tag to be opened, which is not on the list, and therefore all the text until the next tag will be deleted. a better behavior in this case would be to just convert that < to &lt;. you even ought to do this for

a < b

too, despite the fact that b is a valid tag, because who the hell leaves the closing angle bracket out of their HTML tag?

creating the correct regexes for this is an exercise left to the reader.