Access control elements from customvalidator client script

Question

 I have a javascript function that is called on button click event. The problem is that the identifier will not access the control unless i provide the complete path location which resides inside a Listview control.

Path location.ListView1_ctrl5_Label1  Also,the client identifier is not recognised? '<%= Label1.ClientID %>'

function validateUser(sender, args) {
         
                var pr = document.getElementById('Label1').innerText;
                var valid = true;