NHI number validation or whatever

Yeah NHI number validation!  There is an exciting document here that the Ministry of Health have supplied which specifies what makes an NHI number valid or not.

Here's a method y'all.

public bool ValidateNhi(string nhi)
{
    bool valid = false;
    const string validNhiChars = "ABCDEFGHJKLMNPQRSTUVWXYZ";
                
    nhi = nhi.ToUpper();
    // must be seven chars long
    if (nhi.Length == 7)
    {
        // must not contain I or O
        if (!nhi.Contains("I") && !nhi.Contains("O"))
        {
            // last four must be integer
            int parsed;
            if (int.TryParse(nhi.Substring(3), out parsed))
            {
                // multiply each character by 8 minus its index and then sum em all together
                // e.g. first char * 7 + second char * 6 + third char * 5 etc
                int result = 0;
                int counter = 7;
                foreach (var character in nhi.Substring(0, 6)) // exclude last character
                {
                    int parsedCharacter = counter > 4 ? validNhiChars.IndexOf(character) + 1 : (int)Char.GetNumericValue(character);
                    result = result + (counter * parsedCharacter);
                    counter--;
                }
                // create a checksum by mod 11
                int checksum = result % 11;
                int checkDidgit = 11 - checksum;
                if (checkDidgit == 10)
                    checkDidgit = 0;

                // last value in nhi number must equal check digit
                valid = (int)Char.GetNumericValue(nhi.Last()) == checkDidgit;
            }
        }
    }

    return valid;
}

I can't deal with the excitement!