Google app engine - Bugs in the code
<p>My code is this:</p>

<pre><code>import webapp2
import re
form = """&lt;html&gt;
&lt;form method=post&gt;
&lt;input name="username"&gt;&lt;div id=div&gt;%s&lt;/div&gt;&lt;br&gt;
&lt;input name="password"&gt;&lt;div id=div&gt;%s&lt;/div&gt;&lt;br&gt;
&lt;input name="verify"&gt;&lt;div id=div&gt;%s&lt;/div&gt;&lt;br&gt;
&lt;input name="email"&gt;&lt;div id=div&gt;%s&lt;/div&gt;
&lt;input type="submit"&gt;
&lt;/form&gt;
&lt;/html&gt;"""
class MainPage(webapp2.RequestHandler):
  def get(self):
      self.response.headers['Content-Type'] = 'text/html'
      self.response.write(form % ('','','',''))

  def reply(self,user,password,verify,email):
      if not user and password and verify and email:
        self.redirect('/broski')
      else:
        self.response.write(form % (user,password,verify,email))

  def post(self):
      self.usererror = ''
      if not re.compile(r"^[a-zA-Z0-9_-]{3,20}$").match(self.request.get('username')):
       self.usererror = "That's not a valid username."
      self.passworderror = ''
      if not re.compile(r"^.{3,20}$").match(self.request.get('password')):
        self.passworderror = "That wasn't a valid password."
      self.verifyerror = ''
      if self.request.get('verify') != self.request.get('password') and self.request.get('password') != '':
        self.verifyerror = "Your passwords didn't match."
      self.emailerror = ''
      if not re.compile(r"^[\S]+@[\S]+\.[\S]+$").match(self.request.get('email')):
        self.emailerror = "That's not a valid email."
      self.reply(usererror,passworderror,verifyerror,emailerror)

class ThankYo(webapp2.RequestHandler):
  def get(self):
      self.response.write('Welcome, ' + self.request.get('username')+'!')

app = webapp2.WSGIApplication([('/', MainPage),('/broski', ThankYo)],
                              debug=True)
</code></pre>

<p>It is literally my first time writing a code for GAE(And im new to python..) larger than 10 lines and, ofcourse, he broke.
I found several bugs but to absolutely on help as the website still doesnt even load.
Help regarding the code is highly appreciated. 
Update: I updated it a little bit more(Changed inheritance of ThankYo) and now the form successfully loads, as soon as i post the data ofcourse it crashes...</p>
