IPv6 validation

  ipv6: function (val, req, attribute) {
    if (typeof val != 'string')
      return false;

    // regex to check that each hextet is valid
    var er = /^[0-9a-f]+$/;
    // ipv6 hextets are delimited by colon
    hextets = val.split(':');

    // check 1: ipv6 should contain only one consecutive colons
    colons = val.match(/::/);
    if (colons != null && val.match(/::/g).length > 1)
      return false;

    // check 2: ipv6 should not be ending or starting with colon
    //          edge case: not with consecutive colons
    if (val[0] == ':' && (colons == null || (colons != null && colons.index != 0)))
      return false;
    if (val[val.length - 1] == ':' && (colons == null || (colons != null && colons.index != val.length - 2)))
      return false;

    // check 3: ipv6 should contain no less than 3 sector
    //         minimum ipv6 addres - ::1
    if (3 > hextets.length)
      return false;

    // check 4: ipv6 should contain no more than 8 sectors
    //         only 1 edge case: when first or last sector is ommited
    var isEdgeCase = (hextets.length == 9 && colons != null && (colons.index == 0 || colons.index == val.length - 2));
    if (hextets.length > 8 && !isEdgeCase)
      return false;

    // check 5: ipv6 should contain exactly one consecutive colons if it has less than 8 sectors
    if (hextets.length != 8 && colons == null)
      return false;

    for (let i = 0; i < hextets.length; i++) {
      const element = hextets[i];

      if (element.length == 0)
        continue;

      // check 6: all of hextets should contain numbers from 0 to f (in hexadecimal)
      if (!er.test(element))
        return false;

      // check 7: all of hextet values should be less then ffff (in hexadeimal)
      //          checking using length of hextet. lowest invalid value's length is 5.
      //          so all valid hextets are length of 4 or less
      if (element.length > 4)
        return false;
    }
    return true;
  }