Regular expression that matches valid IPv6 addresses

Answer 1

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 FindIPv6InFile(string filePath)
    {
        Char ch;
        StringBuilder sbIPv6 = new StringBuilder();
        List listIPv6 = new List();
        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 FindIPv6InText(string text)
    {
        StringBuilder sbIPv6 = new StringBuilder();
        List listIPv6 = new List();

        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;
        }
    }

}