function check() {

    var s = document.forms['abo'].elements['email'].value;
    if(s == "email@domain.com") return false;
    
    var a = false;
    var res = false;
    
    if(typeof(RegExp) == 'function') {
        var b = new RegExp('abc');
        if(b.test('abc') == true){a = true;}
    }

    if(a == true) {
        reg = new RegExp('^([a-zA-Z0-9\\-\\.\\_]+)'+
            '(\\@)([a-zA-Z0-9\\-\\.]+)'+
            '(\\.)([a-zA-Z]{2,6})$');
        res = (reg.test(s));
    }
    else {
        res = (s.search('@') >= 1 &&
            s.lastIndexOf('.') > s.search('@') &&
            s.lastIndexOf('.') >= s.length-5);
    }
    
    if(!res) alert ("Das ist keine gültige E-Mail-Adresse.");
    return(res);
}