A quick and dirty yet powerful JS router.

router.js

// requires underscore.js for `_.clone`, although come to think about it could be done without it.

// The router module.
var router = (function () {

  // Get the location array and filter out empty strings.
  var path = window.location.pathname.split('/').filter(Boolean)
      // The param arg stack.
    , arg_stack = [];

  // Main match function. Which is called by all api's to match a certain path.
  // with params and regular expression.
  var _match = function (path, param, regHash, args) {
    var parts = param.split('/')
      , passed = true;

    // For every part of our path to match the window path.
    for (var i = 0, p; p = parts[i]; i++) {
      // A part is optional if its starts with a paran.
      var isOptional = !!p.match(/^\(.+\)$/)
          // The current path to match against.
        , cur = path.shift();

      // Pull up the actual path to match.
      p = p.match(/\(?([^\)]+)\)?/)[1];

      // If its a param.
      if (cur && p.charAt(0) === ':') {
        var argName = p.substring(1);
        if (regHash[argName] && !cur.match(regHash[argName])) {
          if (isOptional) {
            path.unshift(cur);
            continue;
          } else {
            passed  = false;
            break;
          }
        }
        args[argName] = cur;
        continue;
      }

      // Its not a param then its must match the current path.
      if (p !== cur) {
        // If path part to match is optional then we unshift the current
        // location path we're matching against.
        if (isOptional) {
          if (cur) path.unshift(cur);
        } else {
          // fail.
          passed = false;
          break;
        }
      }
    }

    return passed ? path : null;
  };

  // Wraps every api call of our module and abstracts the common
  // argument juggling.
  var api = function (call) {
    return function (param, regHash, fns) {
      if (typeof regHash === 'function' || Array.isArray(regHash)) {
        fns = regHash;
        regHash = {};
      }
      var fn;
      if (Array.isArray(fns)) {
        fn = function () {
          var args = arguments;
          fns.forEach(function (f) {
            f.apply(null, args);
          });
        };
      } else {
        fn = fns;
      }
      return call(param, regHash, fn);
    };
  };

  // Just match part of the location path.
  // Takes care of creating a scope (context for other calls).
  var scope = function (param, regHash, fn) {
    var args = arg_stack[arg_stack.length - 1] || {};
    arg_stack.push(args);

    var passed = _match(_.clone(path), param, regHash, args)
      , _path = path;

    if (passed) {
      path = passed;
      fn();
    }

    // Back up after we finished executing our context function.
    path = _path;
    arg_stack.pop();
  };

  // This is a terminal match. Check that all path parts is succesfully
  // matched.
  var match = function (param, regHash, fn) {
    var args = _.clone(arg_stack[arg_stack.length - 1])
      , passed = _match(_.clone(path), param, regHash, args);


    if (passed && !passed.length) {
      fn(args);
    }
  };

  // Just like the scope function but don't have to do post exec
  // stuff.
  var part = function (param, regHash, fn) {
     var args = _.clone(arg_stack[arg_stack.length - 1])
       , passed = _match(_.clone(path), param, regHash, args);

     if (passed) {
       fn(args);
     }
  };

  return {
    scope: api(scope)
  , match: api(match)
  , part: api(part)
  };
})();

usage.js

// path: '/en/sections/123abc/1'
var scope = router.scope, match = router.match, part = router.part;

scope('(:locale)', {locale: /^[a-z]{2}$/g}, function (args) {
  scope('sections', function () {
    part(':section_id', function (args) {
      console.log('init something awesome to do with section %s', args.section_id);
    });
    scope(':section_id', function () {
      match(':section_no', {section_no: /[0-9]+/}, function (args) {
        console.log('some action to do with section #%d', args.section_no);
      });
    });
  });
});