How do you match only valid roman numerals with a regular expression?

Question

Thinking about my other problem, i decided I can\'t even create a regular expression that will match roman numerals (let alone a context-free grammar that will generate them

Answer 1

Fortunately, the range of numbers is limited to 1..3999 or thereabouts. Therefore, you can build up the regex piece-meal.

Each of those parts will deal with the vagaries of Roman notation. For example, using Perl notation:

 = m/(CM|DC{0,3}|CD|C{1,3})?/;

Repeat and assemble.

Added: The can be compressed further:

 = m/(C[MD]|D?C{0,3})/;

Since the 'D?C{0,3}' clause can match nothing, there's no need for the question mark. And, most likely, the parentheses should be the non-capturing type - in Perl:

 = m/(?:C[MD]|D?C{0,3})/;

Of course, it should all be case-insensitive, too.

You can also extend this to deal with the options mentioned by James Curran (to allow XM or IM for 990 or 999, and CCCC for 400, etc).

 = m/(?:[IXC][MD]|D?C{0,4})/;