<script type="text/javascript">

function validate_form()

{

    valid = true;
    exampleregex = ""; //"get it from google"

    if (document.myForm1.example.value == "" ){

        alert ("Empty");

        valid = false;

    }

    else if (!(exampleregex.test(document.myForm1.example.value))) {

        alert("Bad Example");

        valid = false;

    }

        return valid;

}

</script>

<form method="POST" name="myForm1" onsubmit="return validate_form()" action="">

<input name="example" type="text"/>
<input type="submit" value="Process"/>
</form>