The IsEmail Function check if the text in the Element (textbox) is Email or not . Please note
the parameter passed is a Textbox Element. if u want to pass text. u can avoid line 1
inside the function and change line3 to regex.test(elem);


function IsEmail(elem) 
{
    var field = elem.value;
    var regex = new RegExp("^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$");
    return regex.test(field);
}

Usage

if(IsEmail(document.getElementById("<%=txtPrimaryEMail.ClientID%>")))
{
     alert("Valid EMail");
}