Lengthen the pre code fence so backtick runs can't close it early

markdownify/__init__.py

@@ -702,7 +702,13 @@ def convert_pre(self, el, text, parent_tags):
         else:
             raise ValueError('Invalid value for strip_pre: %s' % self.options['strip_pre'])
 
-        return '\n\n```%s\n%s\n```\n\n' % (code_language, text)
+        # Use a fence long enough that no backtick run inside the code can close
+        # it early (CommonMark section 4.5: the closing fence must have at least
+        # as many backticks as the opening).
+        max_backticks = max((len(match) for match in re.findall(re_backtick_runs, text)), default=0)
+        fence = '`' * max(3, max_backticks + 1)
+
+        return '\n\n%s%s\n%s\n%s\n\n' % (fence, code_language, text, fence)
 
     def convert_q(self, el, text, parent_tags):
         return '"' + text + '"'

tests/test_conversions.py

@@ -311,6 +311,15 @@ def test_pre():
     assert md("<p>foo</p>\n<pre>bar</pre>\n</p>baz</p>", sub_symbol="^") == "\n\nfoo\n\n```\nbar\n```\n\nbaz"
 
 
+def test_pre_backticks():
+    # A backtick run inside the code must not close the fence early: the fence
+    # needs at least one more backtick than the longest run in the content.
+    assert md('<pre>foo\n```\nbar</pre>') == '\n\n````\nfoo\n```\nbar\n````\n\n'
+    assert md('<pre><code>```</code></pre>') == '\n\n````\n```\n````\n\n'
+    assert md('<pre>a````b</pre>') == '\n\n`````\na````b\n`````\n\n'
+    assert md('<pre>plain</pre>') == '\n\n```\nplain\n```\n\n'
+
+
 def test_q():
     assert md('foo <q>quote</q> bar') == 'foo "quote" bar'
     assert md('foo <q cite="https://example.com">quote</q> bar') == 'foo "quote" bar'