split on backslash, pop to get the last result, and remove the ")
'url("http://localhost/image/user/temp/n9cec42c939ab461cac7852c267413b1b.jpg")'.split('/').pop().replace('\")', '')
Voorbeeld
Of als RegExp precies jouw ding is:
var url = 'url("http://localhost/image/user/temp/n9cec42c939ab461cac7852c267413b1b.jpg")';
url.match(/.+\/(.+)\"\)/)[1];//file name