Refer the below code.
HTML
<asp:TextBox runat="server" ID="txtmail" type="enroll" value="" placeholder="enroll"></asp:TextBox>
<asp:Button runat="server" ID="btnsubmit" type="submit" Text="Submit" value="Submit"
OnClick="Validate" />
Code
protected void Validate(object sender, EventArgs e)
{
string valueToValidate = txtmail.Text.Trim();
bool SevenDigitNumber = Regex.IsMatch(valueToValidate, "^\\d{7}$");
if (SevenDigitNumber || valueToValidate == "")
{
//Do your stuff.
}
else
{
//Do your stuff.
}
}