i need to throw error in java script

i need output like this [email protected]

var eml=document.getElementById('ctl00_ContentPlaceHolder3_textemail').value;

if(eml=="")

{

alert("Email id can not be blank");

return false;

}

var emailPat = /^(\".*\"|[A-Za-z]\w*)@(\[\d{1,3}(\.\d{1,3}){3}]|[A-Za-z]\w*(\.[A-Za-z]\w*)+)$/;

var emailid=document.getElementById('ctl00_ContentPlaceHolder3_textemail').value;

var matchArray = emailid.match(emailPat);

if (matchArray == null)

{

alert("Your email address seems incorrect. Please try again.");

return false;

}