More Bad Input

Now that the from_roman() function works properly with good input, it’s time to fit in the last piece of the puzzle: making it work properly with bad input. That means finding a way to look at a string and determine if it’s a valid Roman numeral. This is inherently more difficult than validating numeric input in the to_roman() function, but you have a powerful tool at your disposal: regular expressions. (If you’re not familiar with regular expressions, now would be a good time to read the regular expressions chapter.)

As you saw in Case Study: Roman Numerals, there are several simple rules for constructing a Roman numeral, using the letters M, D, C, L, X, V ...