From 85540da9303287cc13fdce95971a23c9799226f3 Mon Sep 17 00:00:00 2001
From: Nick Pope <nick@nickpope.me.uk>
Date: Wed, 7 Jul 2021 23:26:06 +0100
Subject: [PATCH] Refs #32499 -- Fixed escaped metacharacters in
 simplify_regex().

---
 django/contrib/admindocs/utils.py |  6 +++++-
 tests/admin_docs/test_views.py    | 28 ++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/django/contrib/admindocs/utils.py b/django/contrib/admindocs/utils.py
index b3a0526874..1dc30a3960 100644
--- a/django/contrib/admindocs/utils.py
+++ b/django/contrib/admindocs/utils.py
@@ -144,7 +144,11 @@ unnamed_group_matcher = _lazy_re_compile(r'\(')
 
 def replace_metacharacters(pattern):
     """Remove unescaped metacharacters from the pattern."""
-    return pattern.replace('^', '').replace('$', '').replace('?', '')
+    return re.sub(
+        r'((?:^|(?<!\\))(?:\\\\)*)(\\?)([?^$])',
+        lambda m: m[1] + m[3] if m[2] else m[1],
+        pattern,
+    )
 
 
 def replace_named_groups(pattern):
diff --git a/tests/admin_docs/test_views.py b/tests/admin_docs/test_views.py
index 73426a3f8f..bf11ae5be5 100644
--- a/tests/admin_docs/test_views.py
+++ b/tests/admin_docs/test_views.py
@@ -409,6 +409,34 @@ class AdminDocViewFunctionsTests(SimpleTestCase):
             (r'a???', '/a'),
             # Multiple mixed metacharacters.
             (r'^a/?$', '/a/'),
+            # Escaped single metacharacters.
+            (r'\^a', r'/^a'),
+            (r'\\^a', r'/\\a'),
+            (r'\\\^a', r'/\\^a'),
+            (r'\\\\^a', r'/\\\\a'),
+            (r'\\\\\^a', r'/\\\\^a'),
+            (r'a\$', r'/a$'),
+            (r'a\\$', r'/a\\'),
+            (r'a\\\$', r'/a\\$'),
+            (r'a\\\\$', r'/a\\\\'),
+            (r'a\\\\\$', r'/a\\\\$'),
+            (r'a\?', r'/a?'),
+            (r'a\\?', r'/a\\'),
+            (r'a\\\?', r'/a\\?'),
+            (r'a\\\\?', r'/a\\\\'),
+            (r'a\\\\\?', r'/a\\\\?'),
+            # Escaped mixed metacharacters.
+            (r'^a\?$', r'/a?'),
+            (r'^a\\?$', r'/a\\'),
+            (r'^a\\\?$', r'/a\\?'),
+            (r'^a\\\\?$', r'/a\\\\'),
+            (r'^a\\\\\?$', r'/a\\\\?'),
+            # Adjacent escaped metacharacters.
+            (r'^a\?\$', r'/a?$'),
+            (r'^a\\?\\$', r'/a\\\\'),
+            (r'^a\\\?\\\$', r'/a\\?\\$'),
+            (r'^a\\\\?\\\\$', r'/a\\\\\\\\'),
+            (r'^a\\\\\?\\\\\$', r'/a\\\\?\\\\$'),
         )
         for pattern, output in tests:
             with self.subTest(pattern=pattern):