Specifed argument was out of range of valid values. Parameter name: i

Please my code:

Specified argement was out of range of valid values. Parameter name i:

That was my error please see the code below:

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if (FileUpload1.HasFile)
{
string filepath = FileUpload1.PostFile.FileName;
string pat = @"\\(?:.+) \\ (.+) \. (.+)";
Regex r = new Regex(pat);

// run

Match m = r.Match(filepath);
string file_ext = m.Group[2].Captures[0].ToString(); // error on this
line
string filename = m.Group[1].Captures[0].ToString();
string file = filename + "." + file_ext;
}

if (FileUpload1.HasFile)
{
string filepath = FileUpload1.PostFile.FileName;
string pat = @"\\(?:.+) \\ (.+) \. (.+)";
Regex r = new Regex(pat);

// run

Match m = r.Match(filepath);
string file_ext = m.Group[2].Captures[0].ToString(); // error on this line
string filename = m.Group[1].Captures[0].ToString();
string file = filename + "." + file_ext;
}
What is causing the error and how do I fix it?