expressjs.com/en/api/app-param.jade

section
  h3(id='app.param') app.param([name], callback)
  
  p.
    Map logic to route parameters. For example when <code>:user</code>
    is present in a route path you may map user loading logic to automatically
    provide <code>req.user</code> to the route, or perform validations
    on the parameter input.

  p.
    The following snippet illustrates how the <code>callback</code>
    is much like middleware, thus supporting async operations, however
    providing the additional value of the parameter, here named as <code>id</code>.
    An attempt to load the user is then performed, assigning <code>req.user</code>,
    otherwise passing an error to <code>next(err)</code>.

  +js.
    app.param('user', function(req, res, next, id){
      User.find(id, function(err, user){
        if (err) {
          next(err);
        } else if (user) {
          req.user = user;
          next();
        } else {
          next(new Error('failed to load user'));
        }
      });
    });

  p.
    Alternatively you may pass only a <code>callback</code>, in which
    case you have the opportunity to alter the <code>app.param()</code> API.
    For example the <a href="http://github.com/visionmedia/express-params">express-params</a>
    defines the following callback which allows you to restrict parameters to a given
    regular expression. 

  p.
    This example is a bit more advanced, checking if the second argument is a regular
    expression, returning the callback which acts much like the "user" param example.

  +js.
    app.param(function(name, fn){
      if (fn instanceof RegExp) {
        return function(req, res, next, val){
          var captures;
          if (captures = fn.exec(String(val))) {
            req.params[name] = captures;
            next();
          } else {
            next('route');
          }
        }
      }
    });

  p.
    The method could now be used to effectively validate parameters, or also
    parse them to provide capture groups:

  +js.
    app.param('id', /^\d+$/);

    app.get('/user/:id', function(req, res){
      res.send('user ' + req.params.id);
    });

    app.param('range', /^(\w+)\.\.(\w+)?$/);

    app.get('/range/:range', function(req, res){
      var range = req.params.range;
      res.send('from ' + range[1] + ' to ' + range[2]);
    });