How to parse a URL?

The RFC (see appendix B) provides a regular expression to parse the URI parts:

^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
 12            3  4          5       6  7        8 9

where

scheme    = $2
authority = $4
path      = $5
query     = $7
fragment  = $9

---

Example:

function parse_url(url) {
    var pattern = RegExp("^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\\?([^#]*))?(#(.*))?");
    var matches =  url.match(pattern);
    return {
        scheme: matches[2],
        authority: matches[4],
        path: matches[5],
        query: matches[7],
        fragment: matches[9]
    };
}
console.log(parse_url("http://www.somesite.se/blah/sdgsdgsdgs"));

gives

Object
    authority: "www.somesite.se"
    fragment: undefined
    path: "/blah/sdgsdgsdgs"
    query: undefined
    scheme: "http"

DEMO

EDIT (2020): In modern browsers, you can use the built-in URL Web API.

https://developer.mozilla.org/en-US/docs/Web/API/URL/URL

var url = new URL("http://www.somesite.se/blah/sdgsdgsdgs");
var pathname = url.pathname; // returns /blah/sdgsdgsdgs

---

Instead of relying on a potentially unreliable* regex, you should instead use the built-in URL parser that the JavaScript DOM API provides:

var url = document.createElement('a');
url.href = "http://www.example.com/some/path?name=value#anchor";

That's all you need to do to parse the URL. Everything else is just accessing the parsed values:

url.protocol; //(http:)
url.hostname; //(www.example.com)
url.pathname; //(/some/path)
url.search; // (?name=value)
url.hash; //(#anchor)

In this case, if you're looking for /blah/sdgsdgsdgs, you'd access it with url.pathname

Basically, you're just creating a link (technically, anchor element) in JavaScript, and then you can make calls to the parsed pieces directly. (Since you're not adding it to the DOM, it doesn't add any invisible links anywhere.) It's accessed in the same way that values on the location object are.

(Inspired by this wonderful answer.)

EDIT: An important note: it appears that Internet Explorer has a bug where it omits the leading slash on the pathname attribute on objects like this. You could normalize it by doing something like:

 url.pathname = url.pathname.replace(/(^\/?)/,"/");

Note: *: I say "potentially unreliable", since it can be tempting to try to build or find an all-encompassing URL parser, but there are many, many conditions, edge cases and forgiving parsing techniques that might not be considered or properly supported; browsers are probably best at implementing (since parsing URLs is critical to their proper operation) this logic, so we should keep it simple and leave it to them.