const symbols =
  '\\u0020-\\u0026\\u0028-\\u002F\\u003A-\\u0040\\u005B-\\u0060\\u007B-\\u007E\\u00A0-\\u00BF\\u00D7\\u00F7';
const lowers = 'a-z\\u00DF-\\u00F6\\u00F8-\\u00FF';
const uppers = 'A-Z\\u00C0-\\u00D6\\u00D8-\\u00DE';

const reUpper = new RegExp('^[^' + lowers + ']+$');
const reRoom = new RegExp('[' + symbols + ']');
const reRelax = new RegExp('([^' + uppers + '])([' + uppers + ']*)([' + uppers + '])(?=[^' + uppers + ']|$)', 'g');
const reFill = new RegExp('[' + symbols + ']+(.|$)', 'g');

function relax(_match: string, before: string, acronym: string, caps: string): string {
  return before + ' ' + (acronym ? acronym + ' ' : '') + caps;
}

export function snakeCase(value: string): string {
  let s = value == null ? '' : String(value);
  if (reUpper.test(s)) {
    s = s.toLowerCase();
  }
  if (!reRoom.test(s)) {
    s = s.replace(reRelax, relax);
  }
  s = s.toLowerCase();
  s = s.replace(reFill, (_match, next) => (next ? '_' + next : ''));
  return s.replace(/'/g, '');
}