how to retrieve value at the specified index.

// first, get the name of the file without the extsension
string filename = System.IO.Path.GetFileNameWithoutExtension(@"c:\somePath\x_y 1-2-3_2.txt");
// you should now have "x_y 1-2-3_2" as the file name

// find the last underscore
int pos = filename.LastIndexOf("_");

// retrieve all of the data after the underscore
string part = "";
if (pos >= 0 && pos <= filename.Length -1)
{
    part = filename.Substring(pos);
}

// parse it into an integer
int value;
if (Int32.TryParse(part, out value))
{
    // do something with value...
}
else
{
    // error
}

string s = "Bank_Transactions 2011-07-17_2.txt";
s.SubString(s.LastIndexOf('_') +1);
s = s.Replace(".txt", String.Empty);