old_contents: import re
  
  from django.conf import settings
  from django.http import HttpResponseRedirect
  
  
  # Matches things like
  #   /en
  #   /en/
  #   /en/foo/bar (can be anything after the first trailing slash)
  # But not
  #   /en-gb
  # because the fallback language code is not followed immediately by a slash.
  FALLBACK_PREFIX_PATTERN = re.compile(
      r'^/(?P<lang>{langs})(?:/?|/.+)$'.format(
          langs='|'.join(settings.FALLBACK_LANGUAGE_PREFIXES.keys()),
      ),
      re.UNICODE,
  )
  
  
  class LocaleFallbackMiddleware:
      """Redirect entries in ``settings.FALLBACK_LANGUAGE_PREFIXES`` to a
      valid language prefix.
      """
      response_redirect_class = HttpResponseRedirect
  
      def process_request(self, request):
          if not settings.USE_I18N:
              return
          match = FALLBACK_PREFIX_PATTERN.match(request.path_info)
          if not match:
              return
          lang = match.group('lang')
          fallback = settings.FALLBACK_LANGUAGE_PREFIXES[lang]
          path = request.get_full_path().replace(lang, fallback, 1)
          return self.response_redirect_class(path)
  
new_contents: import re
  
  from django.conf import settings
  from django.core.urlresolvers import get_script_prefix
  from django.http import HttpResponseRedirect
  
  
  # Matches things like
  #   /en
  #   /en/
  #   /en/foo/bar (can be anything after the first trailing slash)
  # But not
  #   /en-gb
  # because the fallback language code is not followed immediately by a slash.
  FALLBACK_PREFIX_PATTERN = re.compile(
      r'^/(?P<lang>{langs})(?:/?|/.+)$'.format(
          langs='|'.join(settings.FALLBACK_LANGUAGE_PREFIXES.keys()),
      ),
      re.UNICODE,
  )
  
  
  class LocaleFallbackMiddleware:
      """Redirect entries in ``settings.FALLBACK_LANGUAGE_PREFIXES`` to a
      valid language prefix.
      """
      response_redirect_class = HttpResponseRedirect
  
      def process_request(self, request):
          if not settings.USE_I18N:
              return
          match = FALLBACK_PREFIX_PATTERN.match(request.path_info)
          if not match:
              return
          lang = match.group('lang')
          fallback = settings.FALLBACK_LANGUAGE_PREFIXES[lang]
          script_prefix = get_script_prefix()
          path = request.get_full_path().replace(
              script_prefix + lang, script_prefix + fallback, 1,
          )
          return self.response_redirect_class(path)
  
subject: Prepend script prefix when replacing lang code