Private Sub ComboBox1_TextChanged(sender As Object, e As EventArgs) Handles _
  ComboBox1.TextChanged
  Me.Button1.Enabled = Regex.IsMatch(Me.ComboBox1.Text, "^[A-Za-z0-9]{1,9}$")
End Sub

Private Sub ComboBox1_KeyPress(sender As Object, e As KeyPressEventArgs) _
  Handles ComboBox1.KeyPress
  If e.KeyChar Like "[!A-Za-z0-9]" AndAlso e.KeyChar <> vbBack Then
    My.Computer.Audio.PlaySystemSound(Media.SystemSounds.Beep)
    e.Handled = True
  End If
End Sub