The RUT has 8 digits plus a validation number or letter (xx.xxx.xxx-z) and algorithm for verification (as per my knowledge) is as follows:

1. Remove all characters except digits and last char which can be digit or letter “K”
2. Pad with zeros (“0″) string until it is exactly 9 symbols long
3. Multiply, from left to right, first 8 digits with following factors: 3, 2, 7, 6, 5, 4, 3, 2 and sum those results (let’s call it total)
4. Find difference: 11 – (total % 11) (let’s call it rest)
5. Determine control char by:
   5.1 If rest = 11, than control char “0” (zero)
   5.2 If rest = 10, than control char “K”
   5.3 Otherwise, rest is control char
6. If the original control char (9^th char) is equal to calculated control char, RUT is valid.

C# code for this could be like this:

/// <summary>
/// Routine for checking of RUT correctness
/// </summary>
/// <param name="rut">RUT to check</param>
/// <returns>true if RUT is valid</returns>
/// <remarks>Only numbers and optional "K" at the end of string are expected
/// </remarks>
public static bool IsRutOk(string rut)
{
    const string RutRegex = "[0-9]+K?";
    Regex RegExRut = new Regex(RutRegex, RegexOptions.Compiled |
    RegexOptions.IgnoreCase);
    int[] coefs = {3, 2, 7, 6, 5, 4, 3, 2};

    // In case that rut is padded with spaces
    rut = rut.Trim().ToUpperInvariant();

    if (!RegExRut.IsMatch(rut)) { return false; }

    if (rut.Length > 9) { return false; }

    // If shorter than 9 characters (8 + control char) ...
    while (rut.Length < 9) { rut = "0" + rut; }

    int total = 0;

    for (int index = 0; index < rut.Length - 1; index++)
    {
         char curr = rut.Substring(index, 1).ToCharArray()[0];
         total += coefs[index]*(curr - '0');
    }

    int rest = 11 - (total%11);

    if (rest == 11) rest = 0;

    if ((rest == 10) && rut.EndsWith("K")) { return true; }

    if (rut.Substring(rut.Length - 1, 1).ToCharArray()[0] == ('0' + rest))
    {
        return true;
    }

    return false;
}