I need to add a confirm prompt to step 1 of the reg process


I have added a large number of custom fields to step 1 of the reg process in membership. My client has expressed a desire to amend this so that if a user makes a mistake with the standard registration details (email, username, password etc) then the custom field entries are lost. I have tried but can’t think of a way to repopulate the fields with the submitted form values but think a confirm prompt such as ‘Are you sure you have entered your details correctly’ will perhaps get round this.

Can anybody suggest a way (hook with function perhaps) of adding a confirm prompt to the submit action on registration step 1 to achieve this?