How to validate UserName that should start with characters followed by underscore and then followed by characters but no special chacracters

using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Text.RegularExpressions;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs ex)
        {

            string Content1 = "a_b";
            string Content2 = "a_@";
            string pattern = @"[A-Za-z]_[A-Za-z]";


            Regex.IsMatch(Content1, pattern); // will return true
            Regex.IsMatch(Content2, pattern); // will return false


        }
    }


}