<script language="JavaScript1.2">
function checkpostal(){
var re5digit=/^\d{5}$/ //regular expression defining a 5 digit number
if (document.myform.myinput.value.search(re5digit)==-1) //if match failed
alert("Please enter a valid 5 digit number inside form")
}
</script>
<form name="myform">
<input type="text" name="myinput" size=15>
<input type="button" onClick="checkpostal()" value="check">
</form>
function checkpostal(){
var re5digit=/^\d{5}$/ //regular expression defining a 5 digit number
if (document.myform.myinput.value.search(re5digit)==-1) //if match failed
alert("Please enter a valid 5 digit number inside form")
}
</script>
<form name="myform">
<input type="text" name="myinput" size=15>
<input type="button" onClick="checkpostal()" value="check">
</form>