Have a look at their documentation, there is a line about it:
Regular expressions may also be used, and can be useful if you have very specific restraints, for example the following would match "GET /commits/71dbb9c" as well as "GET /commits/71dbb9c..4c084f9".
app.get(/^\/commits\/(\w+)(?:\.\.(\w+))?$/, function(req, res){
var from = req.params[0];
var to = req.params[1] || 'HEAD';
res.send('commit range ' + from + '..' + to);
});
This should solve your problem :)