A Pleasing Symmetry

Converting a string from a Roman numeral to an integer sounds more difficult than converting an integer to a Roman numeral. Certainly there is the issue of validation. It’s easy to check if an integer is greater than 0, but a bit harder to check whether a string is a valid Roman numeral. But we already constructed a regular expression to check for Roman numerals, so that part is done.

That leaves the problem of converting the string itself. As we’ll see in a minute, thanks to the rich data structure we defined to map individual Roman numerals to integer values, the nitty-gritty of the from_roman() function is as ...