Sensitive credit card number omitter

private static readonly int CreditCardLengthMin = 13;
private static readonly int CreditCardLengthMax = 19;

public static string OmitSensitiveData(string str)
{
    var start = -1; // start index of the card number
    var count = 0; // number of characters in the card number
    var nextSeparator = -1;

    StringBuilder builder = new StringBuilder(str);
    for (int i = 0; i < str.Length; i++)
    {
        char c = builder[i];
        bool expectingSeparator = nextSeparator == i;
        if (expectingSeparator) nextSeparator += 5;
        if (char.IsNumber(c))
        {
            if (start == -1) {
                start = i;
                nextSeparator = i + 4;
            }
            count++;                                    // increment number count
            if (count == CreditCardLengthMin) // have the minimum amount of characters
            {
                // reached the minimum size of a credit card number
                // we will mask it up to here. if it has more then it will be masked
                // as its being parsed
                OmitNumbers(builder, start, i + 1);
                continue;
            } else if (count > CreditCardLengthMin) // have more than the minimum
            {
                // omit the number
                OmitNumber(builder, i);
                if (count >= CreditCardLengthMax)
                {
                    // we've hit the end of the line
                    // no break so start and count will be reset
                } else continue; // haven't reached the maximum yet
            } else continue; // haven't hit a min yet, don't reset
        } else if (start != -1                           // has started
            && (c.Equals(' ') || c.Equals('-'))         // is acceptable character
            && expectingSeparator)
        {
            continue; // acceptable
        }
        // non number or acceptable character, reset
        start = -1;
        count = 0;
        nextSeparator = -1;
    }
    return builder.ToString();
}

private static void OmitNumber(StringBuilder builder, int index)
{
    if (char.IsNumber(builder[index])) builder[index] = 'X';
}

private static void OmitNumbers(StringBuilder builder, int start, int end)
{
    for (int i = start; i < end; i++)
    {
        OmitNumber(builder, i);
    }
}