the code:

// String.isEmail Method v2.0
// by Jonas Galvez (jonas@onrelease.org)

String.prototype.isEmail = function() {
    var ref = arguments.callee;
    if(this.indexOf("@") == -1) return false;
    if(!isNaN(this.charAt(0))) return false;
    var email, user, domain, user_dots, domain_dots;
    if((email = this.split("@")).length == 2) { 
        if((domain = email[1]).split(".").pop().length > 3) return false;

   // update 27 Mar 2003 
   // David Pérez Ortuño (david@ideas-shop.com)       
   if(domain.split(".").length < 2) return false;
   // end update
       
       
        if((user = email[0]).indexOf(".") && domain.indexOf(".")) {     
      
            if(domain.lastIndexOf(".") > domain.length-3) return false;
      
            for(var c, t, i = (user_dots = user.split(".")).length; i--;) {
                c = user_dots[i]; t = !ref.$_text.call(c, "-", ".", "_");
                if(t || !isNaN(c)) return false;
            };
            for(var c, t, i = (domain_dots = domain.split(".")).length; i--;) {
                c = domain_dots[i]; t = !ref.$_text.call(c, "-", ".");
                if(t || !isNaN(c)) return false;
            };
        } else return false;
    } else return false;
    return true;
};
String.prototype.isEmail.$_punctuation = function() {
    if(this == "") return false;
    for(var i = arguments.length; i--;) {
        if(this.indexOf(arguments[i]) == 0) return false;
        if(this.indexOf(arguments[i]) == this.length-1) return false;
    };
    return true;
};
String.prototype.isEmail.$_text = function() {
    var ref = arguments.caller;
    if(!ref.$_punctuation.apply(this, arguments)) return false;
    var others = arguments; var checkOthers = function(str) {
        for(var i = others.length; i--;) if(str == others[i]) return true;
        return false;
    };
    for(var c, alpha, num, i = this.length; i--;) {
        c = this.charAt(i).toLowerCase();
        alpha = (c <= "z") && (c >= "a");
        num = (c <= "9") && (c >= "0");
        if(!alpha && !num && !checkOthers(c)) return false;
    };
    return true;
}; 
ASSetPropFlags(String.prototype, "isEmail", 1);

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