Roman Numeral Converter

Questions

Why is "IIII" rejected for 4?

It isn't standard. Standard Roman numerals use IV, IX, XL, XC, CD and CM so no symbol repeats more than three times. The tool only reads and writes the standard form.

Why is the limit 3999?

M (1000) is the largest standard symbol, and it repeats at most three times. 3999 is MMMCMXCIX. Larger numbers used a line over the numeral, which plain text can't show.

Does case matter?

No. "mcmxciv" and "MCMXCIV" both work, and the output is always in capitals.

Is there a Roman numeral for 0?

No. Roman numerals have no zero, so the range starts at 1.