Lua function check if ipv4 or ipv6 or string

Question

I'd like to have a function that I can pass a whitespace trimmed string to and it will return
0 for error (not a string) 1 for ipv4 2 for ipv6 3 for a string thats not an ip.

Ipv6 has these rules:

Ipv6 is represented by 8 groups of 16-bit hexadecimal values separated by colons (:)
The hexadecimal digits are case-insensitive
Abbreviation rules:
1: Omit leading zeroes in a 16-bit value
2: Replace one or more groups of consecutive zeroes by a double colon

wiki example showing 3 ways that are all the same ipv6:

fe80:0000:0000:0000:0202:b3ff:fe1e:8329
fe80:0:0:0:202:b3ff:fe1e:8329
fe80::202:b3ff:fe1e:8329 

I'm reasonably sure for ipv4 you just check for three . then check the string is all
numbers and the .'s are counted as numbers and the last check for just a string
would be at the end of an if statement so if its not ipv4/6 and its a string then
it returns 3

Answer 1

Mike's solution is good, but it can be improved on in several ways. In its current form it doesn't get to ipv6 address check, but it's easy to fix. The ipv6 check fails on things like "1050!0!0+0-5@600$300c#326b" and "1050:0:0:0:5:600:300c:326babcdef" (recognizing both as valid addresses) and "1050:::600:5:1000::" (recognizing it as string).

Here is the improved version (IPv4 are assumed to be decimal numbers and IPv6 are assumed to be hexadecimal numbers):

function GetIPType(ip)
  local R = {ERROR = 0, IPV4 = 1, IPV6 = 2, STRING = 3}
  if type(ip) ~= "string" then return R.ERROR end

  -- check for format 1.11.111.111 for ipv4
  local chunks = {ip:match("^(%d+)%.(%d+)%.(%d+)%.(%d+)$")}
  if #chunks == 4 then
    for _,v in pairs(chunks) do
      if tonumber(v) > 255 then return R.STRING end
    end
    return R.IPV4
  end

  -- check for ipv6 format, should be 8 'chunks' of numbers/letters
  -- without leading/trailing chars
  -- or fewer than 8 chunks, but with only one `::` group
  local chunks = {ip:match("^"..(("([a-fA-F0-9]*):"):rep(8):gsub(":$","$")))}
  if #chunks == 8
  or #chunks < 8 and ip:match('::') and not ip:gsub("::","",1):match('::') then
    for _,v in pairs(chunks) do
      if #v > 0 and tonumber(v, 16) > 65535 then return R.STRING end
    end
    return R.IPV6
  end

  return R.STRING
end

The script to check:

local IPType = {[0] = "Error", "IPv4", "IPv6", "string"}
local ips = {
    "128.1.0.1", -- ipv4
    "223.255.254.254", -- ipv4
    "999.12345.0.0001", -- invalid ipv4
    "1050:0:0:0:5:600:300c:326b", -- ipv6
    "1050!0!0+0-5@600$300c#326b", -- string
    "1050:0:0:0:5:600:300c:326babcdef", -- string
    "1050:0000:0000:0000:0005:0600:300c:326b", -- ipv6
    "fe80:0000:0000:0000:0202:b3ff:fe1e:8329", -- ipv6
    "fe80:0:0:0:202:b3ff:fe1e:8329", -- ipv6
    "fe80::202:b3ff:fe1e:8329", -- ipv6
    "1050:::600:5:1000::", -- contracted ipv6
    "::", -- ipv6
    "::1", -- ipv6
    "::1::", -- string
    "129.garbage.9.1", -- string
    "xxx127.0.0.0", -- error
    "xxx1050:0000:0000:0000:0005:0600:300c:326b", -- string
    129.10 -- error
}
for k,v in pairs(ips) do
    print(v, IPType[GetIPType(v)])
end

And the output:

128.1.0.1   IPv4
223.255.254.254 IPv4
999.12345.0.0001    string
1050:0:0:0:5:600:300c:326b  IPv6
1050!0!0+0-5@600$300c#326b  string
1050:0:0:0:5:600:300c:326babcdef    string
1050:0000:0000:0000:0005:0600:300c:326b IPv6
fe80:0000:0000:0000:0202:b3ff:fe1e:8329 IPv6
fe80:0:0:0:202:b3ff:fe1e:8329   IPv6
fe80::202:b3ff:fe1e:8329    IPv6
1050:::600:5:1000:: IPv6
::  IPv6
::1 IPv6
::1::   string
129.garbage.9.1 string
xxx127.0.0.0    string
xxx1050:0000:0000:0000:0005:0600:300c:326b  string
129.1   Error

Updated on 9/6/2018 to add handling of garbage before/after addresses and checking for contracted ipv6, which allows for fewer than 8 groups with one empty group of two consecutive colons.