autocomplete: Identify when the user intends a channel link autocomplete

For this commit we temporarily intercept the query at the
AutocompleteField widget, to avoid invoking the widgets that are
still unimplemented. That lets us defer those widgets' logic to a
separate later commit.

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -49,6 +49,9 @@ extension ComposeContentAutocomplete on ComposeContentController {
       } else if (charAtPos == ':') {
         final match = _emojiIntentRegex.matchAsPrefix(textUntilCursor, pos);
         if (match == null) continue;
+      } else if (charAtPos == '#') {
+        final match = _channelLinkIntentRegex.matchAsPrefix(textUntilCursor, pos);
+        if (match == null) continue;
       } else {
         continue;
       }
@@ -67,6 +70,10 @@ extension ComposeContentAutocomplete on ComposeContentController {
         final match = _emojiIntentRegex.matchAsPrefix(textUntilCursor, pos);
         if (match == null) continue;
         query = EmojiAutocompleteQuery(match[1]!);
+      } else if (charAtPos == '#') {
+        final match = _channelLinkIntentRegex.matchAsPrefix(textUntilCursor, pos);
+        if (match == null) continue;
+        query = ChannelLinkAutocompleteQuery(match[1] ?? match[2]!);
       } else {
         continue;
       }
@@ -166,6 +173,32 @@ final RegExp _emojiIntentRegex = (() {
     + r')$');
 })();
 
+final RegExp _channelLinkIntentRegex = () {
+  // Similar reasoning as in _mentionIntentRegex.
+  const before = r'(?<=^|\s|\p{Punctuation})';
+
+  // TODO(upstream): maybe use duplicate-named capture groups for better readability?
+  //   https://github.com/dart-lang/sdk/issues/61337
+  return RegExp(unicode: true,
+    before
+    + r'#'
+    // As Web, match both '#channel' and '#**channel'. In both cases, the raw
+    // query is going to be 'channel'. Matching the second case ('#**channel')
+    // is useful when the user selects a channel from the autocomplete list, but
+    // then starts pressing "backspace" to edit the query and choose another
+    // option, instead of clearing the entire query and starting from scratch.
+    + r'(?:'
+      // Case '#channel': right after '#', reject whitespace as well as '**'.
+      + r'(?!\s|\*\*)(.*)'
+      + r'|'
+      // Case '#**channel': right after '#**', reject whitespace.
+      // Also, make sure that the remaining query doesn't contain '**',
+      // otherwise '#**channel**' (which is a complete channel link syntax) and
+      // any text followed by that will always match.
+      + r'\*\*(?!\s)((?:(?!\*\*).)*)'
+    + r')$');
+}();
+
 /// The text controller's recognition that the user might want autocomplete UI.
 class AutocompleteIntent<QueryT extends AutocompleteQuery> {
   AutocompleteIntent({

lib/widgets/autocomplete.dart

@@ -45,7 +45,8 @@ class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends
   }
 
   void _handleControllerChange() {
-    final newQuery = widget.autocompleteIntent()?.query;
+    var newQuery = widget.autocompleteIntent()?.query;
+    if (newQuery is ChannelLinkAutocompleteQuery) newQuery = null; // TODO(#124)
     // First, tear down the old view-model if necessary.
     if (_viewModel != null
         && (newQuery == null