Regex for Multiple Check Box

why regex?
you can use client side validation using javascript or jquary
jQuery see if any or no checkboxes are selected[^]
and you may need do a server side validation, then

CheckBox[] checkboxes = new CheckBox[] { CheckBox1, CheckBox2, CheckBox3, CheckBox4 };
if (!checkboxes.Any(c => c.Checked))
{
    // show error 
}

It has got nothing to do with regex, just use javascript to validate before submission
check-box-validation-for-atleast-one-check-box-should-cheked-in-asp-net[^]