private void txtUsername_Validating(object sender, CancelEventArgs e)
{
System.Text.RegularExpressions.Regex rEMail = new System.Text.RegularExpressions.Regex("^[a-zA-Z0-9_]");
if (UserID.Text.Length > 0)
{
if (!rEMail.IsMatch(UserID.Text))
{
MessageBox.Show("only letters,numbers and underscore is allowed", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
UserID.SelectAll();
e.Cancel = true;
}
}
}