This problem sounds like it suffers from a "Lack of Specification". You indicate that you want to convert back and forth from plaintext to HTML. However, there's a reason why there's two formats - they do different things. I ran into this when attempting to design a Document::Template that would handle PDF, Excel, and other formats. PDF and Excel are sufficiently different that it makes no sense, and HTML is even worse. A better question would be How can I convert a HTML table into fixed-width columns and back again? This is an easily solvable problem. (I could have a solution in 20 minutes and under 250 characters ... golf, anyone?)
in reply to HTML <=> Text convertion
Now, mentioning PDF brings up another idea - there are HTML => PDF converters and PDF => plaintext converters. There are also plaintext => PDF converters, but no PDF => HTML converters (that I'm aware of). A big problem with converting from XXX => HTML is that HTMl is a non-deterministic format. I find it easier to consider HTML a "hinting format" instead of a "defining format" (like PDF). Browsers, to be compliant, are free to implement whatever they want, so long as they implement something. (This is how you have HTML-x compliant browsers for the blind.)
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Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.