Ensure that mention behavior is consistent cross-platform

Signal/src/ViewControllers/ConversationView/Cells/OWSMessageBubbleView.m

@@ -709,7 +709,7 @@ NS_ASSUME_NONNULL_BEGIN
     NSString *text = displayableText.displayText;
     
     NSError *error1;
-    NSRegularExpression *regex1 = [[NSRegularExpression alloc] initWithPattern:@"@\\w*" options:0 error:&error1];
+    NSRegularExpression *regex1 = [[NSRegularExpression alloc] initWithPattern:@"@[0-9a-fA-F]*" options:0 error:&error1];
     OWSAssertDebug(error1 == nil);
     NSSet<NSString *> *knownUserIDs = LKAPI.userIDCache[thread.uniqueId];
     NSMutableArray<NSValue *> *mentions = [NSMutableArray new];

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -3773,13 +3773,11 @@ typedef enum : NSUInteger {
     [self.typingIndicators didStartTypingOutgoingInputInThread:self.thread];
     NSUInteger currentEndIndex = textView.text.length - 1;
     unichar lastCharacter = [textView.text characterAtIndex:currentEndIndex];
-    NSMutableCharacterSet *allowedCharacters = NSMutableCharacterSet.lowercaseLetterCharacterSet;
-    [allowedCharacters formUnionWithCharacterSet:NSCharacterSet.uppercaseLetterCharacterSet];
     if (lastCharacter == '@') {
         NSArray<NSString *> *userIDs = [LKAPI getUserIDsFor:@"" in:self.thread.uniqueId];
         self.mentionStartIndex = (NSInteger)currentEndIndex + 1;
         [self.inputToolbar showUserSelectionViewFor:userIDs in:self.thread];
-    } else if (![allowedCharacters characterIsMember:lastCharacter]) {
+    } else if ([NSCharacterSet.whitespaceAndNewlineCharacterSet characterIsMember:lastCharacter]) {
         self.mentionStartIndex = -1;
         [self.inputToolbar hideUserSelectionView];
     } else {