Adding value to input field with jQuery

You can simply use

 $(this).prev().val("hello world");

JSFiddle Demo

You have to escape [ and ]. Try this:

$('.button').click(function(){
    var fieldID = $(this).prev().attr("id");
    fieldID = fieldID.replace(/([\[\]]+)/g, "\\$1");
    $('#' + fieldID).val("hello world");
});

Demo: http://jsfiddle.net/7RJtf/1/

You can do it as below.

$(this).prev('input').val("hello world");

Live Demo