<script type="text/javascript">
var efilter=/^\w+[\+\.\w-]*@([\w-]+\.)*\w+[\w-]*\.([a-z]{2,4}|\d+)$/i

function checkit(e){
var retval=efilter.test(e.value)
if (retval==false){
alert("Please enter a valid email address.")
e.select()
}
return retval
}

</script>

<form>
<input name="txtemail" type="text"> <input type="submit" onClick="return checkit(this.form.txtemail)" value="Submit" />

</form>