username having one underscore and should not allow any special characters

public bool IsAlphaNumericWithUnderscore(string input)
{
    return Regex.IsMatch(input, "^[a-zA-Z0-9_]+$");
}

bool result = input.All(c=>Char.IsLetterOrDigit(c) || c=='_');

Regex rx = new Regex("^[a-zA-Z0-9_]*$");
<pre lang="cs">if (rx.IsMatch(UserName.Text))
                    {
                        Valid = true;
                    }
                    else
                    {
MessageBox.Show("Special characters cannot be submitted as a part of the Username (except Underscore).\nPlease re enter the User name and click Button.");
                        UserName.Text = "";
                        UserName.Focus();
                        return;
                    }