text box enter only characters

private void txtbank_TextChanged(object sender, TextChangedEventArgs e)
       {

           if (System.Text.RegularExpressions.Regex.IsMatch(txtbank.Text, "^[a-zA-Z]+$"))
           {
           }
           else
           {
               txtbank.Text = txtbank.Text.Remove(txtbank.Text.Length -1);

               //MessageBox.Show("Enter only Alphabets");
           }
       }