in the following function, it is prefixed and suffixed with / and /.
var emailExpression = /\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*/;
var element = document.getElementById('<%=txtFirstName.ClientID %>');
Above function is assuming that you have a txtFirstName TextBox for which the value needs to be validated. If that value will have the correct email id format, this will return true else false.