Ensure that mention behavior is consistent cross-platform

src/org/thoughtcrime/securesms/conversation/ConversationActivity.java

@@ -2756,7 +2756,7 @@ public class ConversationActivity extends PassphraseRequiredActionBarActivity
         List<Tuple2<String, String>> users = LokiAPI.Companion.getUsers("", threadId, userDatabase);
         mentionStartIndex = currentEndIndex + 1;
         userSelectionView.show(users, threadId);
-      } else if (Character.getType(lastCharacter) != Character.UPPERCASE_LETTER && Character.getType(lastCharacter) != Character.LOWERCASE_LETTER) {
+      } else if (Character.isWhitespace(lastCharacter)) {
         mentionStartIndex = -1;
         userSelectionView.hide();
       } else {

src/org/thoughtcrime/securesms/conversation/ConversationItem.java

@@ -777,7 +777,7 @@ public class ConversationItem extends LinearLayout
   }
 
   private SpannableString highlightMentions(CharSequence text, boolean isGroupThread) {
-    Pattern pattern = Pattern.compile("@\\w*");
+    Pattern pattern = Pattern.compile("@[0-9a-fA-F]*");
     Matcher matcher = pattern.matcher(text);
     ArrayList<Range<Integer>> mentions = new ArrayList<>();
     if (matcher.find() && isGroupThread) {