markdown: Update bugdown emoticon translation logic to match frontend.

This PR solves some of the parity issues in the emoticon translation
logic. I was unable to find a way of matching only one of the
lookaround groups, so we still have some inconsistency (see
testcase). The approach of having another check while converting just
for this seemed like an inefficient way, so I've left that last change
as it is.

zerver/fixtures/markdown_test_cases.json

@@ -381,6 +381,21 @@
       "text_content": ":)",
       "translate_emoticons": true
     },
+    {
+      "name": "translate_emoticons_at_sentence_end",
+      "input": "Translate this :).",
+      "expected_output": "<p>Translate this <span class=\"emoji emoji-1f603\" title=\"smiley\">:smiley:</span>.</p>",
+      "text_content": "Translate this \ud83d\ude03.",
+      "translate_emoticons": true
+    },
+    {
+      "name": "translate_emoticons_between_symbols",
+      "input": "Translate this !:)?",
+      "expected_output": "<p>Translate this !<span class=\"emoji emoji-1f603\" title=\"smiley\">:smiley:</span>?</p>",
+      "marked_expected_output": "<p>Translate this !:)?</p>",
+      "text_content": "Translate this !\ud83d\ude03?",
+      "translate_emoticons": true
+    },
     {
       "name": "random_emoji_1",
       "input": ":airplane:",

zerver/lib/emoji.py

@@ -28,7 +28,10 @@ EMOTICON_CONVERSIONS = {
 
 possible_emoticons = EMOTICON_CONVERSIONS.keys()
 possible_emoticon_regexes = map(re.escape, possible_emoticons)  # type: ignore # AnyStr/str issues
-emoticon_regex = '(?<![^\s])(?P<emoticon>(' + ')|('.join(possible_emoticon_regexes) + '))(?![\S])'  # type: ignore # annoying
+terminal_symbols = ',.;?!()\\[\\] "\'\\n\\t'  # type: str # from composebox_typeahead.js
+emoticon_regex = ('(?<![^{0}])(?P<emoticon>('.format(terminal_symbols)
+                  + ')|('.join(possible_emoticon_regexes)  # type: ignore # AnyStr/str issues
+                  + '))(?![^{0}])'.format(terminal_symbols))
 
 # Translates emoticons to their colon syntax, e.g. `:smiley:`.
 def translate_emoticons(text: Text) -> Text: