WIP Validate URL with Regex

app/main/forms.py

@@ -2285,7 +2285,10 @@ class UrlForm(StripWhitespaceForm):
         "URL",
         validators=[
             DataRequired(message="Cannot be empty"),
-            Regexp(regex="^https.*", message="Must be a valid https URL"),
+            Regexp(
+                regex=r"^https:\/\/docs\.notifications\.service\.gov\.uk\/(python|ruby|node|net|java|php|rest-api)\.html#.+",
+                message="Must be a valid https URL, pointing to a section within the GOV.UK Notify API docs.",
+            ),
         ],
     )
 

tests/app/main/views/test_index.py

@@ -402,3 +402,24 @@ def test_POST_guidance_api_documentation_section(client_request):
             section_tag="send-a-file-by-email",
         ),
     )
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "",  # empty string
+        "https://docs.notifications.service.gov.uk/python.html",  # no section
+        "https://docs.payments.service.gov.uk/making_payments/#creating-a-payment",  # URL is notfor Notify's docs
+    ],
+)
+def test_POST_guidance_api_documentation_section_with_incorrect_url(client_request, url):
+    page = client_request.post(
+        "main.guidance_api_documentation_section",
+        _data={"url": "https://docs.notifications.service.gov.uk/python.html#send-a-file-by-email"},
+        _expected_status=200,
+    )
+
+    assert (
+        "Must be a valid https URL, pointing to a section within the GOV.UK Notify API docs."
+        in page.select_one(".govuk-error-message").text
+    )