code: function getSubstitution(matched, str, position, captures, namedCaptures, replacement) {
      var tailPos = position + matched.length;
      var m = captures.length;
      var symbols = SUBSTITUTION_SYMBOLS_NO_NAMED;
      if (namedCaptures !== undefined) {
        namedCaptures = toObject(namedCaptures);
        symbols = SUBSTITUTION_SYMBOLS;
      }
      return $replace.call(replacement, symbols, function (match, ch) {
        var capture;
        switch (ch.charAt(0)) {
          case '$': return '$';
          case '&': return matched;
          case '`': return str.slice(0, position);
          case "'": return str.slice(tailPos);
          case '<':
            capture = namedCaptures[ch.slice(1, -1)];
            break;
          default: // \d\d?
            var n = +ch;
            if (n === 0) return match;
            if (n > m) {
              var f = floor(n / 10);
              if (f === 0) return match;
              if (f <= m) return captures[f - 1] === undefined ? ch.charAt(1) : captures[f - 1] + ch.charAt(1);
              return match;
            }
            capture = captures[n - 1];
        }
        return capture === undefined ? '' : capture;
      });
    }
docs: https://tc39.github.io/ecma262/#sec-getsubstitution