Series string comparer

public int Compare (string x, string y)
{
    if (x == y)
        return 0;

    int xi = 0;
    int yi = 0;

    while (xi < x.Length && yi < y.Length) {
        while(xi < x.Length && char.IsWhiteSpace(x[xi])) xi++;
        while(yi < y.Length && char.IsWhiteSpace(y[yi])) yi++;

        if(char.IsDigit(x[xi]) && char.IsDigit(y[yi]))
        {
            string xDigits = "";

            do
            {
                xDigits += x[xi];
                xi++;
            }
            while(xi < x.Length && char.IsDigit(x[xi]));

            string yDigits = "";

            do
            {
                yDigits += y[yi];
                yi++;
            }
            while(yi < y.Length && char.IsDigit(y[yi]));

            int xVal = ParseDigits(xDigits);
            int yVal = ParseDigits(yDigits);

            if(xVal > yVal) return 1;
            else if (xVal < yVal) return -1;
        }
        else if(x[xi] != y[yi])
        {
            return x[xi].CompareTo(y[yi]);
        }
        else
        {
            xi++;
            yi++;
        }
    }

    return 0;
}

public static int ParseDigits (string digits)
{
    string newDigits = "";

    foreach (char c in digits) {
        if (c >= '0' && c <= '9')
            newDigits += c;
        else if (c >= '\uFF10' && c <= '\uFF19')
            newDigits += (char)(c - '\uFF10' + '0');
        else if (c >= '\u0660' && c <= '\u0669')
            newDigits += (char)(c - '\u0660' + '0');
        else if (c >= '\u09E6' && c <= '\u09EF')
            newDigits += (char)(c - '\u09E6' + '0');
    }

    if (newDigits == "")
        return 0;
    else
        return int.Parse (newDigits);
}