A rudimentary URL parser in javascript.

url_parser.js

var route = function(format) {

  var splitIntoParts = function(url) {
    var urlString = url.match(/^[^\?]*/) || [""]
    return urlString[0].split('/');
  };
  var formatParts = splitIntoParts(format); // Split the format into parts for easy reuse

  var splitIntoQueryParams = function(url) {
    var paramString = url.match(/\?.*$/) || [""];
    return paramString[0].split('&');
  };

  var populateRouteParams = function(instanceParts, params) {
    for(var i = 0; i < formatParts.length; i++) {
      if (formatParts[i].match(/^:/)) // populate params if the current route part is a variable
        params[formatParts[i].replace(/^:/, '')] = instanceParts[i]
      else if (formatParts[i] != instanceParts[i]) // otherwise check for equality
        throw 'Route does not match format: ' + format;
    }
  };

  var populateQueryParams = function(queryParams, params) {
    for(var i = 0; i < queryParams.length; i++) {
      if (queryParams[i].length > 0) {
        queryParam            = queryParams[i].match(/([a-z]*)=([a-z]*)/i); // Split each param around the = sign
        params[queryParam[1]] = queryParam[2]; // Set the 'key' value on params to the query param value.
      }
    }
  };

  return function(instance) {
    var params = {};

    try {
      populateRouteParams(splitIntoParts(instance), params);
      populateQueryParams(splitIntoQueryParams(instance), params);
    }
    catch(e) { // Log the error, then return an empty set of params
      params = {};
      console.log(e);
    }

    return params;
  };
};

var apiRoute    = route('/api/:version/:collection/:id');
var memberRoute = route('/groups/:groupId/users/:memberId/:action');

console.log(apiRoute('/api/v2/geese/5'));
console.log(apiRoute('/api/v1/mallards/1?color=green&gender=male'));
console.log(apiRoute('/groups/5/users/10/subscribe'))

console.log(memberRoute('/groups/5/users/10/subscribe'));
console.log(memberRoute('/groups/6/users/4/ban?reason=profanity'));
console.log(memberRoute('/api/v1/users/10'));

A couple thoughts / potential area for improvement:

• If I had a jQuery, I'd likely want to do something like return $.extend({}, populateRouteParams, populateQueryParams), so I didn't have to pass the params object around like that.
• Right now the query params take precedence over the route params.. I'm not certain if that's right or not? (ie given route /users/:user_id, url /users/1?user_id=2 would return {user_id: 2})
• Matching the query parameters right now (with this: (/([a-z]*)=([a-z]*)/i)) is pretty wrong, as it will only parse simple alphabetic words correctly (so, member_id, or my%20places would break). I'm sure there's a more complete regex out there, but it seemed out of scope for this.)

Some output from running the above in a console:

Object {version: "v2", collection: "geese", id: "5"} Script snippet #1:53
Object {version: "v1", collection: "mallards", id: "1", color: "green", gender: "male"} Script snippet #1:54
Route does not match format: /api/:version/:collection/:id Script snippet #1:43
Object {} Script snippet #1:55
Object {groupId: "5", memberId: "10", action: "subscribe"} Script snippet #1:57
Object {groupId: "6", memberId: "4", action: "ban", reason: "profanity"} Script snippet #1:58
Route does not match format: /groups/:groupId/users/:memberId/:action Script snippet #1:43
Object {}