Preserving Existing Input On Error?

I have a form validication function that checks a variety of form inputs which appears to be working fine. Trouble is that if I deliberately throw it some non-valid entries it clears the form with the data I've already supplied. preserving the input the user has already made so that this doesn't need to be re-input when the user has input invalid data?

Here's my validation code. which checks..

Whether a user has input their full name - entering full name is optional and if not true will negate subsequent checks, i.e. doesn't bother checking any other inputs. Whether a user has supplied both first and last names, fail message pops up if not. Whether a user has suppllied a full name but not an email or telephone number, fail message pops up if not. Whether a user has supplied a valid email address, fail message pops up if not. Whether a user has supplied a valid telephone number, fail message pops up if not. If we've passed all these tests, O.K. - submit.

function check_id(){
var status = false;
var emailRegEx = /^[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}$/i;

[code].....