function OpenWin(mypage,myname,w,h,features) {
     if(screen.width){
     var winl = (screen.width-w)/2;
     var wint = (screen.height-h)/2;
     }else{winl = 0;wint =0;}
     if (winl < 0) winl = 0;
     if (wint < 0) wint = 0;
     var settings = 'height=' + h + ',';
     settings += 'width=' + w + ',';
     settings += 'top=' + wint + ',';
     settings += 'left=' + winl + ',';
     settings += features;
     win = window.open(mypage,myname,settings);
     win.window.focus();
   }
   
function checkIfEmpty(field) {
    if (document.getElementById(field).value != "") {
        return true;
    } else {
        alert(field + " is verplicht");
        document.getElementById(field).focus();
	    return false;
    }
}

function IsEmailValid(checkThisEmail) {
	var myEMailIsValid = true;
	var myAtSymbolAt = document.getElementById(checkThisEmail).value.indexOf('@');
	var myLastDotAt = document.getElementById(checkThisEmail).value.lastIndexOf('.');
	var mySpaceAt = document.getElementById(checkThisEmail).value.indexOf(' ');
	var myLength = document.getElementById(checkThisEmail).length;
	
	
	// at least one @ must be present and not before position 2
	// @yellow.com : NOT valid
	// x@yellow.com : VALID
	
	if (myAtSymbolAt < 1 ) 
	 {myEMailIsValid = false}
	
	// at least one . (dot) afer the @ is required
	// x@yellow : NOT valid
	// x.y@yellow : NOT valid
	// x@yellow.org : VALID
	
	if (myLastDotAt < myAtSymbolAt) 
	 {myEMailIsValid = false}
	
	// at least two characters [com, uk, fr, ...] must occur after the last . (dot)
	// x.y@yellow. : NOT valid
	// x.y@yellow.a : NOT valid
	// x.y@yellow.ca : VALID
	
	if (myLength - myLastDotAt <= 2) 
	 {myEMailIsValid = false}
	
	
	// no empty space " " is permitted (one may trim the email)
	// x.y@yell ow.com : NOT valid
	
	if (mySpaceAt != -1) 
	 {myEMailIsValid = false}
	
	
	if (myEMailIsValid == true) {

		return true;
	} else {
    	alert("email adres is niet correct");
		document.getElementById(checkThisEmail).focus();
		return false;
	}
}