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

In my case, I was trying to find and replace all occurences of roman numbers by one word inside the text, so I couldn't use the start and end of lines. So the @paxdiablo solution found many zero-length matches. I ended up with the following expression:

(?=\b[MCDXLVI]{1,6}\b)M{0,4}(?:CM|CD|D?C{0,3})(?:XC|XL|L?X{0,3})(?:IX|IV|V?I{0,3})

My final Python code was like this:

import re
text = "RULES OF LIFE: I. STAY CURIOUS; II. NEVER STOP LEARNING"
text = re.sub(r'(?=\b[MCDXLVI]{1,6}\b)M{0,4}(?:CM|CD|D?C{0,3})(?:XC|XL|L?X{0,3})(?:IX|IV|V?I{0,3})', 'ROMAN', text)
print(text)

Output:

RULES OF LIFE: ROMAN. STAY CURIOUS; ROMAN. NEVER STOP LEARNING