function getRegex(s) {

  return new RegExp("([&?]+)" + s + "=.*&");

}

​

​

// take foo=bar off

// prints "http://mysite.com?baz=boo"

// this is working correctly

var url = 'http://mysite.com?foo=bar&baz=boo';

console.log(url.replace(getRegex('foo'), "$1"));

​

​

// take foo=bar off

// prints "http://mysite.com?foo=bar

// this is currently not working

var url = 'http://mysite.com?foo=bar';

console.log(url.replace(getRegex('foo'), "$1"));