Numerals

This little utility converts valid Roman numerals to Arabic, and vice-versa.

Conversion from Roman numerals

There are a few rules to observe in checking the validity of a roman number:

• Having two subtraction in a row is illegal: IXC does not equal 91 (C - (X - I))
• Having four similar numerals in a row is illegal: 400 should be written as CD (D - C), rather than CCCC
• But IIII is fine: This is typically used by watchmakers to make the reading of the number 4 easy to read upside down.
• L and D cannot be repeated: LL should be C, and DD should be M
• If a certain sequence can be represented with another symbol, it is illegal: LC should be L

Although unicode characters exist, Apostrophus and Vinculum are not supported.

Conversion from Arabic numerals

Due to the rule limiting 4 consecutive Roman numerals, the greatest Arabic numeral is 3999.

Usage

From the command line, use so:

$ numerals IXC
Invalid sequence

$ numerals XCIX
99

$ numerals 198
CXCVIII

Unicode input and output are also supported:

$ numerals ⅬⅡ
52

$ numerals -u 1994
ⅯⅭⅯⅩⅭⅣ

$ numerals --unicode 1994
ⅯⅭⅯⅩⅭⅣ

Known issues

Any value ending in 9, short of 9 itself, will accept invalid input or return invalid output.
For instance, the value XCIX is accepted and returned as 99, although it should reject it and return IC.

However, according to this infoplease page:

According to librarians at NIST, while MIM is more convenient, MCMXCIX is
favored because of earlier precedents with numbers such as 49 (written as
XLIX rather than IL); however, the librarians point out that purists use
neither MIM nor MCMXCIX, opting instead for MCMXCVIIII. The ancient Romans,
they explain, did not use the 20th century convention of IX for the number
nine.

-The Editors

This means that in classical Roman numerals, 4 identical consecutive numerals are allowed, which would change the allowed range from 3999 to 4999.