Javascript Regular Expression validation For Email

 function validate(email) {
        var reg = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
        return reg.test(email);
    }

 Here you need to pass the email address as parameter.

CodeProject  function validateEmail(sender, args) {
        var isValid = 1;
        var strEmails = document.getElementById("<%= txtEmailAddress.ClientID %>").value;
        var emails = strEmails.split(";");
        for (i = 0; i < emails.length; i++) {
            if (!validate(trim(emails[i]))) {
                isValid = -1;
                break;
            }
        }
        if (isValid == -1) {
            args.IsValid = false;
        }
        else {
            args.IsValid = true;
        }
    }

txtEmailAddress is the rich text box. This function is the asp custom validator function