Django urlresolvers reverse for dynamically generated ids

I've got to test URLs that look like this: http://ift.tt/1KWEu9i but I'm not sure how to say this to the reverse() function. This is my urls.py from django.conf.urls import patterns, url from payment import views from payment.msn import *

urlpatterns = patterns('',
    url(r'^$', views.index, name='index'),
    url(r'^msnstats/', MsnStats.finish_transaction, name='finish_transaction'),
    url(r'^pay/(?P<payment_id>\w+)/$', views.pay, name='pay')
)

And this is what the relevant part of my views.py looks like:

def pay(request, payment_id):
    try:
        plan=PaymentPlan.objects.get(payment_id=payment_id)

The payment_id is generated for each plan, so I first create a plan, get its payment_id from the database, and somehow call it. I'm just not sure how to use reverse.