Webapp2 routing regex failing - regex

Question

I've been having trouble getting webapp2 to recognise a path:
app = webapp2.WSGIApplication([
(r'/welcome\?username=([a-zA-Z0-9_-]{3,20})$', WelcomePage), # FAILS
(r'/', MainPage),
], debug=True)
I inserted some prints in the calling function (MainPage) to check the regex:
welcome_path = "/welcome?username=" + username
print welcome_path #debug stuff: check to if regex matches
m = re.match(r'/welcome\?username=([a-zA-Z0-9_-]{3,20})$', welcome_path)
print m.groups(0)
self.redirect(welcome_path) #
Based on the server log it looks ok:
INFO 2015-04-29 20:07:23,667 module.py:788] default: "GET / HTTP/1.1" 200 1287
/welcome?username=fred # printed by debug lines above
(u'fred',) # indicates match and captured group
INFO 2015-04-29 20:07:34,588 module.py:788] default: "POST / HTTP/1.1" 302 -
INFO 2015-04-29 20:07:34,614 module.py:788] default: "GET /welcome?username=fred HTTP/1.1" **404** 154
Regex from WSGIApplication (a) and inserted in calling function are identical - copy-pasted here:
r'/welcome\?username=([a-zA-Z0-9_-]{3,20})$' # a
r'/welcome\?username=([a-zA-Z0-9_-]{3,20})$' # b
For completeness:
class WelcomePage(webapp2.RequestHandler):
def get(self, username):
self.response.write("<h1>Welcome, {}!</h1>".format(username))
But there's a 404 error... Why is the WSGIApplication failing to match the path?
(Note: this isn't production code - just an exercise from a MOOC on Udacity, whose forums have been down for a week...)

Answer 1

This was happening because redirect triggers a GET request, which is matched according to the path: after the hostname, but before the '?', not everything after the hostname as I'd assumed.
So this worked:
app = webapp2.WSGIApplication([
(r'/welcome', WelcomePage),
(r'/', MainPage),
], debug=True)
class WelcomePage(webapp2.RequestHandler):
def get(self):
username = self.request.get('username')
if check_username(username) == "": # returns "" if no error
self.response.write("<h1>Welcome, {}!</h1>".format(username))
else:
self.redirect('/')