smarty: Add a special case for 's at the beginning of the token

When 's is not preceded by anything, it probably means that it comes
after some HTML tag, so it should be converted to a closing quote.

The reference Perl implementation makes the close_class optional and
adds a lookahead check for (\s|s\b) when close_class was not matched.

Let's copy that behavior by removing closeClass lookbehind check from
closingSingleQuotesRegex2.

Fixes Python-Markdown#1305.

docs/change_log/index.md

@@ -3,6 +3,10 @@ title: Change Log
 Python-Markdown Change Log
 =========================
 
+*under development*: version 3.4.4 (a bug-fix release).
+
+* Add a special case for initial 's to smarty extension (#1305).
+
 March 23, 2023: version 3.4.3 (a bug-fix release).
 
 * Restore console script (#1327).

markdown/extensions/smarty.py

@@ -139,7 +139,7 @@
 
 # Single closing quotes:
 closingSingleQuotesRegex = r"(?<=%s)'(?!\s|s\b|\d)" % closeClass
-closingSingleQuotesRegex2 = r"(?<=%s)'(\s|s\b)" % closeClass
+closingSingleQuotesRegex2 = r"'(\s|s\b)"
 
 # All remaining quotes should be opening ones
 remainingSingleQuotesRegex = r"'"

tests/test_syntax/extensions/test_smarty.py

@@ -64,6 +64,10 @@ def test_basic(self):
             '"Ellipsis within quotes..."',
             '<p>&ldquo;Ellipsis within quotes&hellip;&rdquo;</p>'
         )
+        self.assertMarkdownRenders(
+            "*Custer*'s Last Stand",
+            "<p><em>Custer</em>&rsquo;s Last Stand</p>"
+        )
 
     def test_years(self):
         self.assertMarkdownRenders("1440--80's", '<p>1440&ndash;80&rsquo;s</p>')