Javascript RegExp not escaping parenthesis properly. Why?

Question

I had this code that didn't work as I expected. For some reason, it seems that the escaping of the parentheses was not working. This is the original code:

var string = "url('http://localhost/custom/img/image.jpg')"; /* it can be single or double quotes */
var base_url = "http://localhost"; /* this can be any url */
var re = new RegExp("url\((\"|\')" + base_url + "\/(.*)(\"|\')\)"); /* notice the escaped parentheses in two places */
var clean_str = string.replace(re,'$2');
console.log clean_str

No match was found so the result in console was: url('http://localhost/custom/img/image.jpg')

So I change the RegExp line to the one below:

/* I replaced the escaping character with a character set */
var re = new RegExp("url[(](\"|\')" + base_url + "\/(.*)(\"|\')[)]");

result in console: custom/img/image.jpg

And now it works. What am I doing wrong on the first one? I tried some simpler expressions, with trying to escape parentheses, like

var re = new RegExp("url\(");

and got problems like "Unterminated group".

Thanks.