diff --git a/rest_framework/routers.py b/rest_framework/routers.py
index 2ee4b457f..838eebc35 100644
--- a/rest_framework/routers.py
+++ b/rest_framework/routers.py
@@ -254,7 +254,7 @@ class SimpleRouter(BaseRouter):
                 #   so a slash in the beginning will (A) cause Django to give
                 #   warnings and (B) generate URLS that will require using '//'.
                 if not prefix and regex[:2] == '^/':
-                    regex = f'^{regex[2:]}'
+                    regex = '^' + regex[2:]
 
                 initkwargs = route.initkwargs.copy()
                 initkwargs.update({
diff --git a/rest_framework/urlpatterns.py b/rest_framework/urlpatterns.py
index e4b4aedcb..1ed5f89f8 100644
--- a/rest_framework/urlpatterns.py
+++ b/rest_framework/urlpatterns.py
@@ -11,7 +11,7 @@ def _get_format_path_converter(suffix_kwarg, allowed):
         else:
             allowed_cases = "|".join(allowed)
             allowed_pattern = f'(?:{allowed_cases})'
-        suffix_pattern = fr"\.{allowed_pattern}/?"
+        suffix_pattern = r"\.%s/?" % allowed_pattern
     else:
         suffix_pattern = r"\.[a-z0-9]+/?"
 
@@ -102,9 +102,9 @@ def format_suffix_patterns(urlpatterns, suffix_required=False, allowed=None):
         else:
             allowed_cases = "|".join(allowed)
             allowed_pattern = f'({allowed_cases})'
-        suffix_pattern = fr'\.(?P<{suffix_kwarg}>{allowed_pattern})/?$'
+        suffix_pattern = r'\.(?P<%s>%s)/?$' % (suffix_kwarg, allowed_pattern)
     else:
-        suffix_pattern = fr'\.(?P<{suffix_kwarg}>[a-z0-9]+)/?$'
+        suffix_pattern = r'\.(?P<%s>[a-z0-9]+)/?$' % suffix_kwarg
 
     converter_name, suffix_converter = _get_format_path_converter(suffix_kwarg, allowed)
     register_converter(suffix_converter, converter_name)
diff --git a/rest_framework/utils/html.py b/rest_framework/utils/html.py
index d10dbcf8e..c7ede7803 100644
--- a/rest_framework/utils/html.py
+++ b/rest_framework/utils/html.py
@@ -83,7 +83,7 @@ def parse_html_dict(dictionary, prefix=''):
     }
     """
     ret = MultiValueDict()
-    regex = re.compile(fr'^{re.escape(prefix)}\.(.+)$')
+    regex = re.compile(r'^%s\.(.+)$' % re.escape(prefix))
     for field in dictionary:
         match = regex.match(field)
         if not match: