case-insensitive search

i'm perplexing case-insensitive hunt twin strings javascript working.

normally this:

var string="stack  best";
var result= string.search(/best/i);
alert(result);

the /i fall case-insensitive.

but i need hunt second string; but fall works perfect:

var string="stack  best";
var searchstring="best";
var result= string.search(searchstring);
alert(result);

if i supplement /i fall above instance hunt searchstring non-static "searchstring" (next instance working):

var string="stack  best";
var searchstring="best";
var result= string.search(/searchstring/i);
alert(result);

how i grasp this?