code: function GroupRefOrDecChar(text, textLoc) {
    const reference = Number(text.slice(1));
  
    if (reference > 0 && reference <= capturingGroupsCount) {
      return Node({
        type: 'Backreference',
        kind: 'number',
        number: reference,
        reference,
      }, textLoc);
    }
  
    return Char(text, 'decimal', textLoc);
  }
docs: Parses patterns like \1, \2, etc. either as a backreference
  to a group, or a deciaml char code.