Since Roman Numerals only use a handful of characters, I always just use something along these lines:
Search: \b([XIV]{2,})\b
Replace: <span class="romannumeral">\1</span>
That will find you most. (You can add LCDM if you want... but I've yet to see a book really go that high.)
Note #1: To get single character roman numerals, you'll have to be much more careful. I usually split it into different searches, because 'I' is such a common English word.
So something like this:
Search: \b([XV]+)\b
would find more single Roman Numerals.
Note #2: Roman Numerals usually also depend on some further context:
- King Charles V ruled from [...]
- See Chapter I.
- The reference is on p. v.
so I use the specifics of the book to help refine the searches:
Search: Chapter ([XIV]+)
Replace: Chapter <span class="romannumeral">\1</span>