Add linkcheck_case_sensitive configuration option

CHANGES.rst

@@ -48,6 +48,11 @@ Features added
 * #13439: linkcheck: Permit warning on every redirect with
   ``linkcheck_allowed_redirects = {}``.
   Patch by Adam Turner and James Addison.
+* #14046: linkcheck: Add :confval:`linkcheck_case_insensitive` configuration to
+  allow case-insensitive URL comparison for specific URL patterns.
+  This is useful for links to websites that normalise URL casing (for example,
+  GitHub) or case-insensitive servers.
+  Patch by Fazeel Usmani.
 * #13497: Support C domain objects in the table of contents.
 * #13500: LaTeX: add support for ``fontawesome6`` package.
   Patch by Jean-François B.

doc/usage/configuration.rst

@@ -3813,6 +3813,38 @@ and the number of workers to use.
 
    .. versionadded:: 7.3
 
+.. confval:: linkcheck_case_insensitive
+   :type: :code-py:`list` of :code-py:`str`
+   :default: :code-py:`[]`
+
+   A list of regular expressions that match URLs for which the *linkcheck*
+   builder should perform case-insensitive comparisons. This is useful for
+   links to websites that normalise URL casing (for example, GitHub) or
+   servers that are case-insensitive (for example, Windows-based servers).
+
+   By default, *linkcheck* requires the destination URL to match the
+   documented URL case-sensitively. For example, a link to
+   ``http://example.com/PATH`` that redirects to ``http://example.com/path``
+   will be reported as ``redirected``.
+
+   If the URL matches a pattern in this list, such redirects will instead be
+   reported as ``working``.
+
+   For example, to treat all GitHub URLs as case-insensitive:
+
+   .. code-block:: python
+
+      linkcheck_case_insensitive = [
+          r'https://github\.com/.*',
+      ]
+
+   .. note::
+
+      HTML anchor checking is always case-sensitive and is not affected by
+      this setting.
+
+   .. versionadded:: 8.2
+
 .. confval:: linkcheck_rate_limit_timeout
    :type: :code-py:`int`
    :default: :code-py:`300`

sphinx/builders/linkcheck.py

@@ -409,6 +409,9 @@ def __init__(
         self.user_agent = config.user_agent
         self.tls_verify = config.tls_verify
         self.tls_cacerts = config.tls_cacerts
+        self.case_insensitive_patterns: list[re.Pattern[str]] = list(
+            map(re.compile, config.linkcheck_case_insensitive)
+        )
 
         self._session = requests._Session(
             _ignored_redirects=tuple(map(re.compile, config.linkcheck_ignore))
@@ -629,8 +632,29 @@ def _check_uri(self, uri: str, hyperlink: Hyperlink) -> _URIProperties:
         netloc = urlsplit(req_url).netloc
         self.rate_limits.pop(netloc, None)
 
+        # Check if URL should be compared case-insensitively based on patterns
+        is_case_insensitive = any(
+            pattern.match(req_url) for pattern in self.case_insensitive_patterns
+        )
+
+        # Compare URLs, optionally case-insensitively
+        def _normalise_url(url: str) -> str:
+            """Reduces a URL to a normal/equality-comparable form."""
+            normalised_url = url.rstrip('/')
+            if is_case_insensitive:
+                # Only casefold the URL before the fragment; fragments are case-sensitive
+                if '#' in normalised_url:
+                    url_part, fragment = normalised_url.split('#', 1)
+                    normalised_url = url_part.casefold() + '#' + fragment
+                else:
+                    normalised_url = normalised_url.casefold()
+            return normalised_url
+
+        normalised_request_url = _normalise_url(req_url)
+        normalised_response_url = _normalise_url(response_url)
+
         if (
-            (response_url.rstrip('/') == req_url.rstrip('/'))
+            normalised_request_url == normalised_response_url
             or _allowed_redirect(req_url, response_url, self.allowed_redirects)
         ):  # fmt: skip
             return _Status.WORKING, '', 0
@@ -816,6 +840,9 @@ def setup(app: Sphinx) -> ExtensionMetadata:
     app.add_config_value(
         'linkcheck_report_timeouts_as_broken', False, '', types=frozenset({bool})
     )
+    app.add_config_value(
+        'linkcheck_case_insensitive', [], '', types=frozenset({list, tuple})
+    )
 
     app.add_event('linkcheck-process-uri')
 

tests/roots/test-linkcheck-case-check/conf.py

@@ -0,0 +1 @@
+# Empty config for linkcheck case sensitivity tests

tests/roots/test-linkcheck-case-check/index.rst

@@ -0,0 +1,3 @@
+`path1 <http://localhost:7777/path1>`_
+
+`path2 <http://localhost:7777/path2>`_

tests/test_builders/test_build_linkcheck.py

@@ -1439,3 +1439,72 @@ def test_linkcheck_exclude_documents(app: SphinxTestApp) -> None:
         'uri': 'https://www.sphinx-doc.org/this-is-another-broken-link',
         'info': 'br0ken_link matched br[0-9]ken_link from linkcheck_exclude_documents',
     } in content
+
+
+class CapitalisePathHandler(BaseHTTPRequestHandler):
+    """Test server that uppercases URL paths via redirects."""
+
+    protocol_version = 'HTTP/1.1'
+
+    def do_GET(self):
+        if self.path.startswith('/') and len(self.path) > 1 and self.path[1:].islower():
+            # Redirect lowercase paths to uppercase versions
+            self.send_response(301, 'Moved Permanently')
+            self.send_header('Location', self.path.upper())
+            self.send_header('Content-Length', '0')
+            self.end_headers()
+        elif (
+            self.path.startswith('/') and len(self.path) > 1 and self.path[1:].isupper()
+        ):
+            # Serve uppercase paths
+            content = b'ok\n\n'
+            self.send_response(200, 'OK')
+            self.send_header('Content-Length', str(len(content)))
+            self.end_headers()
+            self.wfile.write(content)
+        else:
+            self.send_response(404, 'Not Found')
+            self.send_header('Content-Length', '0')
+            self.end_headers()
+
+
+@pytest.mark.sphinx(
+    'linkcheck',
+    testroot='linkcheck-case-check',
+    freshenv=True,
+)
+@pytest.mark.parametrize(
+    ('case_insensitive_pattern', 'expected_path1', 'expected_path2'),
+    [
+        ([], 'redirected', 'redirected'),  # default: case-sensitive
+        (
+            [r'http://localhost:\d+/.*'],
+            'working',
+            'working',
+        ),  # all URLs case-insensitive
+        (
+            [r'http://localhost:\d+/path1'],
+            'working',
+            'redirected',
+        ),  # only path1 case-insensitive
+    ],
+)
+def test_linkcheck_case_sensitivity(
+    app: SphinxTestApp,
+    case_insensitive_pattern: list[str],
+    expected_path1: str,
+    expected_path2: str,
+) -> None:
+    """Test case-sensitive and case-insensitive URL checking."""
+    app.config.linkcheck_case_insensitive = case_insensitive_pattern
+
+    with serve_application(app, CapitalisePathHandler) as address:
+        app.build()
+
+    content = (app.outdir / 'output.json').read_text(encoding='utf8')
+    rows = [json.loads(x) for x in content.splitlines()]
+    rowsby = {row['uri']: row for row in rows}
+
+    # Verify expected status for each path
+    assert rowsby[f'http://{address}/path1']['status'] == expected_path1
+    assert rowsby[f'http://{address}/path2']['status'] == expected_path2