Reverse symbolic romanize

rsr.js

/*
  I posted this on twitter (on my old/alternate account) some years ago but
  thought I'd preserve it here. Kevlin Henney, in one of his talks, points
  out that the decimal to roman numeral kata is often thought of as a
  numerical or maths problem but that if you think of it as direct symbol
  manipulation (or doing math in unary, which is almost the same thing)
  it's a lot easier. All I did was reverse the ordering of the example in a
  way that I think makes the logic more clear. Doing it this way without
  the grep repetition operators would have been very cumbersome and my guess
  is that that's why the original example was ordered the other way.
  
  https://twitter.com/Whiskey2sday/status/1157215416688578562/
  
  AUTHOR - @WhiskeyTuesday
  LICENCE - WTFPL 3.1 - https://ph.dtf.wtf/u/wtfplv31 for more details.
*/

const rsr = (n) => (
  'I'.repeat(n)
  .replace(/I{1000}/g, 'M')
  .replace(/I{900}/g, 'CM')
  .replace(/I{500}/g, 'D')
  .replace(/I{400}/g, 'CD')
  .replace(/I{100}/g, 'C')
  .replace(/I{90}/g, 'XC')
  .replace(/I{50}/g, 'L')
  .replace(/I{40}/g, 'XL')
  .replace(/I{10}/g, 'X')
  .replace(/I{9}/g, 'IX')
  .replace(/I{5}/g, 'V')
  .replace(/I{4}/g, 'IV')
);