markdown: Update regex for strikethrough.

This uses the correct regex for strikethrough. Also, added
a test to make sure that strikethrough works when it contains
link with whitespace.

Fixes #7596.

zerver/fixtures/markdown_test_cases.json

@@ -188,6 +188,12 @@
       "expected_output": "<p>I <del> like software </del> love hardware</p>",
       "text_content": "I  like software  love hardware"
     },
+    {
+      "name": "strikthrough_link",
+      "input": "~~test http://xx.xx link~~",
+      "expected_output": "<p><del>test <a href=\"http://xx.xx\" target=\"_blank\" title=\"http://xx.xx\">http://xx.xx</a> link</del></p>",
+      "text_content": "test http://xx.xx link"
+    },
     {
       "name": "underscore_disabled",
       "input": "_foo_",

zerver/lib/bugdown/__init__.py

@@ -1395,7 +1395,7 @@ class Bugdown(markdown.Extension):
         # Custom strikethrough syntax: ~~foo~~
         md.inlinePatterns.add('del',
                               markdown.inlinepatterns.SimpleTagPattern(
-                                  r'(?<!~)(\~\~)([^~{0}\n]+?)\2(?!~)', 'del'), '>strong')
+                                  r'(?<!~)(\~\~)([^~\n]+?)(\~\~)(?!~)', 'del'), '>strong')
 
         # Text inside ** must start and end with a word character
         # it need for things like "const char *x = (char *)y"