NET用正则表达式判断字符串长度必须为多长

//--------------------Check the IMEI's format which Must be 15  figure----------------

Regex reg=new Regex(@"^\d{15}$",RegexOptions.None);
if(!reg.IsMatch(this.textBoxIMEI.Text))
{
  labelStatus.Text="IMEI 格式有誤!";
  labelStatus.ForeColor=Color.Red;
  return;
}