Regular expression that matches valid IPv6 addresses

Answer 1

The following will validate IPv4, IPv6 (full and compressed), and IPv6v4 (full and compressed) addresses:

'/^(?>(?>([a-f0-9]{1,4})(?>:(?1)){7}|(?!(?:.*[a-f0-9](?>:|$)){8,})((?1)(?>:(?1)){0,6})?::(?2)?)|(?>(?>(?1)(?>:(?1)){5}:|(?!(?:.*[a-f0-9]:){6,})(?3)?::(?>((?1)(?>:(?1)){0,4}):)?)?(25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9]?[0-9])(?>\.(?4)){3}))$/iD'

Answer 2

Looking at the patterns included in the other answers there are a number of good patterns that can be improved by referencing groups and utilizing lookaheads. Here is an example of a pattern that is self referencing that I would utilize in PHP if I had to:

^(?<hgroup>(?<hex>[[:xdigit:]]{0,4}) # grab a sequence of up to 4 hex digits
                                     # and name this pattern for usage later
     (?<!:::):{1,2})                 # match 1 or 2 ':' characters
                                     # as long as we can't match 3
 (?&hgroup){1,6} # match our hex group 1 to 6 more times
 (?:(?:
    # match an ipv4 address or
    (?<dgroup>2[0-5]|(?:2[0-4]|1{0,1}[0-9]){0,1}[0-9])\.){3}(?&dgroup)
    # match our hex group one last time
    |(?&hex))$

Note: PHP has a built in filter for this which would be a better solution than this pattern.

Regex101 Analysis

Answer 3

I generated the following using python and works with the re module. The look-ahead assertions ensure that the correct number of dots or colons appear in the address. It does not support IPv4 in IPv6 notation.

pattern = '^(?=\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$)(?:(?:25[0-5]|[12][0-4][0-9]|1[5-9][0-9]|[1-9]?[0-9])\.?){4}$|(?=^(?:[0-9a-f]{0,4}:){2,7}[0-9a-f]{0,4}$)(?![^:]*::.+::[^:]*$)(?:(?=.*::.*)|(?=\w+:\w+:\w+:\w+:\w+:\w+:\w+:\w+))(?:(?:^|:)(?:[0-9a-f]{4}|[1-9a-f][0-9a-f]{0,3})){0,8}(?:::(?:[0-9a-f]{1,4}(?:$|:)){0,6})?$'
result = re.match(pattern, ip)
if result: result.group(0)

Answer 4

It sounds like you may be using Python. If so, you can use something like this:

import socket

def check_ipv6(n):
    try:
        socket.inet_pton(socket.AF_INET6, n)
        return True
    except socket.error:
        return False

print check_ipv6('::1') # True
print check_ipv6('foo') # False
print check_ipv6(5)     # TypeError exception
print check_ipv6(None)  # TypeError exception

I don't think you have to have IPv6 compiled in to Python to get inet_pton, which can also parse IPv4 addresses if you pass in socket.AF_INET as the first parameter. Note: this may not work on non-Unix systems.

Answer 5

I'd have to strongly second the answer from Frank Krueger.

Whilst you say you need a regular expression to match an IPv6 address, I'm assuming what you really need is to be able to check if a given string is a valid IPv6 address. There is a subtle but important distinction here.

There is more than one way to check if a given string is a valid IPv6 address and regular expression matching is only one solution.

Use an existing library if you can. The library will have fewer bugs and its use will result in less code for you to maintain.

The regular expression suggested by Factor Mystic is long and complex. It most likely works, but you should also consider how you'd cope if it unexpectedly fails. The point I'm trying to make here is that if you can't form a required regular expression yourself you won't be able to easily debug it.

If you have no suitable library it may be better to write your own IPv6 validation routine that doesn't depend on regular expressions. If you write it you understand it and if you understand it you can add comments to explain it so that others can also understand and subsequently maintain it.

Act with caution when using a regular expression whose functionality you can't explain to someone else.

Answer 6

It is difficult to find a regular expression which works for all IPv6 cases. They are usually hard to maintain, not easily readable and may cause performance problems. Hence, I want to share an alternative solution which I have developed: Regular Expression (RegEx) for IPv6 Separate from IPv4

Now you may ask that "This method only finds IPv6, how can I find IPv6 in a text or file?" Here are methods for this issue too.

Note: If you do not want to use IPAddress class in .NET, you can also replace it with my method. It also covers mapped IPv4 and special cases too, while IPAddress does not cover.

class IPv6
{
    public List<string> FindIPv6InFile(string filePath)
    {
        Char ch;
        StringBuilder sbIPv6 = new StringBuilder();
        List<string> listIPv6 = new List<string>();
        StreamReader reader = new StreamReader(filePath);
        do
        {
            bool hasColon = false;
            int length = 0;

            do
            {
                ch = (char)reader.Read();

                if (IsEscapeChar(ch))
                    break;

                //Check the first 5 chars, if it has colon, then continue appending to stringbuilder
                if (!hasColon && length < 5)
                {
                    if (ch == ':')
                    {
                        hasColon = true;
                    }
                    sbIPv6.Append(ch.ToString());
                }
                else if (hasColon) //if no colon in first 5 chars, then dont append to stringbuilder
                {
                    sbIPv6.Append(ch.ToString());
                }

                length++;

            } while (!reader.EndOfStream);

            if (hasColon && !listIPv6.Contains(sbIPv6.ToString()) && IsIPv6(sbIPv6.ToString()))
            {
                listIPv6.Add(sbIPv6.ToString());
            }

            sbIPv6.Clear();

        } while (!reader.EndOfStream);
        reader.Close();
        reader.Dispose();

        return listIPv6;
    }

    public List<string> FindIPv6InText(string text)
    {
        StringBuilder sbIPv6 = new StringBuilder();
        List<string> listIPv6 = new List<string>();

        for (int i = 0; i < text.Length; i++)
        {
            bool hasColon = false;
            int length = 0;

            do
            {
                if (IsEscapeChar(text[length + i]))
                    break;

                //Check the first 5 chars, if it has colon, then continue appending to stringbuilder
                if (!hasColon && length < 5)
                {
                    if (text[length + i] == ':')
                    {
                        hasColon = true;
                    }
                    sbIPv6.Append(text[length + i].ToString());
                }
                else if (hasColon) //if no colon in first 5 chars, then dont append to stringbuilder
                {
                    sbIPv6.Append(text[length + i].ToString());
                }

                length++;

            } while (i + length != text.Length);

            if (hasColon && !listIPv6.Contains(sbIPv6.ToString()) && IsIPv6(sbIPv6.ToString()))
            {
                listIPv6.Add(sbIPv6.ToString());
            }

            i += length;
            sbIPv6.Clear();
        }

        return listIPv6;
    }

    bool IsEscapeChar(char ch)
    {
        if (ch != ' ' && ch != '\r' && ch != '\n' && ch!='\t')
        {
            return false;
        }

        return true;
    }

    bool IsIPv6(string maybeIPv6)
    {
        IPAddress ip;
        if (IPAddress.TryParse(maybeIPv6, out ip))
        {
            return ip.AddressFamily == AddressFamily.InterNetworkV6;
        }
        else
        {
            return false;
        }
    }

}