Validating a textfield once a checkbox is ticked

Hi
Can anyone help?

I have a simple contact form which I have so far managed to validate, the one function I do not know how to do is this-

If the browser selects the checkbox I need them to fill out a textfield as well(10 digit number to be exact) or the form doesn’t validate, if they de-select or do not select the checkbox I want the form to validate without it.

Hope that makes sense

Thanks MAC

If you want to do this before the submit button is pressed then you will need to use JavaScript. Here is some sample code:

[COLOR=#ff0000]<script Language="JavaScript">
<!-- 
function Blank_TextField_Validator()
{
// Check the value of the element named text_name
// from the form named text_form
if (text_form.text_name.value == "")
{
// If null display and alert box
alert("Please fill in the text field.");
// Place the cursor on the field for revision
text_form.text_name.focus();
// return false to stop further processing
return (false);
}
// If text_name is not null continue processing
return (true);
}
-->
</script>[/COLOR]

<form name="text_form" method="get" action="http://www.codeave.com/html/get.asp" 
[COLOR=#ff0000] onsubmit="return Blank_TextField_Validator()"[/COLOR]>