Add reverse domain name notation property

CHANGELOG.md

@@ -3,6 +3,14 @@
 After upgrading, update your cache file by deleting it or via `tldextract
 --update`.
 
+## unreleased
+
+* Misc.
+  * Add `reverse_domain_name` property, for Reverse Domain Name Notation.
+    See:
+    * https://en.wikipedia.org/wiki/Reverse_domain_name_notation
+    * https://github.com/john-kurkowski/tldextract/issues/342
+
 ## 5.1.3 (2024-11-04)
 
 * Bugfixes

tests/main_test.py

@@ -415,6 +415,38 @@ def test_ipv4_lookalike() -> None:
     )
 
 
+def test_reverse_domain_name_notation() -> None:
+    """Test property `reverse_domain_name`."""
+    assert (
+        tldextract.extract("www.example.com").reverse_domain_name == "com.example.www"
+    )
+    assert (
+        tldextract.extract("www.theregister.co.uk").reverse_domain_name
+        == "co.uk.theregister.www"
+    )
+    assert tldextract.extract("example.com").reverse_domain_name == "com.example"
+    assert (
+        tldextract.extract("theregister.co.uk").reverse_domain_name
+        == "co.uk.theregister"
+    )
+    assert (
+        tldextract.extract("media.forums.theregister.co.uk").reverse_domain_name
+        == "co.uk.theregister.forums.media"
+    )
+    assert (
+        tldextract.extract(
+            "foo.uk.com", include_psl_private_domains=False
+        ).reverse_domain_name
+        == "com.uk.foo"
+    )
+    assert (
+        tldextract.extract(
+            "foo.uk.com", include_psl_private_domains=True
+        ).reverse_domain_name
+        == "uk.com.foo"
+    )
+
+
 def test_bad_kwargs_no_way_to_fetch() -> None:
     """Test an impossible combination of kwargs that disable all ways to fetch data."""
     with pytest.raises(ValueError, match="disable all ways"):

tldextract/tldextract.py

@@ -137,6 +137,25 @@ def ipv6(self) -> str:
                 return debracketed
         return ""
 
+    @property
+    def reverse_domain_name(self) -> str:
+        """Return the Reverse Domain Name Notation.
+
+        Applies a reverse domain name notation to the submitted domain, in which the
+        registered domain is used as the leftmost component. Reverse Domain Name
+        Notation is typically used to organize namespaces for packages and plugins.
+
+        >>> extract("login.example.com").reverse_domain_name
+        'com.example.login'
+
+        >>> extract("login.example.co.uk").reverse_domain_name
+        'co.uk.example.login'
+        """
+        stack = [self.suffix, self.domain]
+        if self.subdomain:
+            stack.extend(reversed(self.subdomain.split(".")))
+        return ".".join(stack)
+
 
 class TLDExtract:
     """A callable for extracting, subdomain, domain, and suffix components from a URL."""