Simple C# Test projects - 2 Index of Longest run in a String

This method can return the zero-based index of longest run in a string. The run is a consecutive sequence of the same characters. If there are more (longest) run with same character then returns with the first index of the run. I used Regular expression which matches patterns with input string.

public static int IndexOfLongestRun(string str)
    {
        StringBuilder pattern = new StringBuilder();
        pattern.Append(@"([a-z])" + new string('*', str.Length - 1));

        pattern=pattern.Replace("*",@"\1");
        Match m = null;
        for (int i = (pattern.Length-7); i > 0; i--)
        {
            pattern = pattern.Remove(pattern.Length-2,1);
            m = Regex.Match(str, pattern.ToString());
            if (m.ToString() != "") break;
        }

        return str.IndexOf(m.ToString());
    }