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 
}

You can implement the technique I explained on my Blog - ASP.NET CHECKBOXLIST CLIENT SIDE VALIDATION USING JAVASCRIPT[^].