5.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Custom Exception</title>
    <script>
        function check_username() {
            var username = document.getElementById('username').value;
            var pattern = /[a-z]|[0-9]/;
            try{
            if(pattern.test(username)==false){
                throw "Enter valid username!";
            }else {
                throw "It is Valid username!:)";
            }
            }catch(err){
                alert(err);
            }
        }
    </script>
</head>
<body>
<p style="font-family: 'calbri';font-size: 12pt; color: blue;">Usename:</p>
<input id = "username" type="text" onchange="check_username();">
</body>
</html>