content [nfc]: Pull out parseUserMention, give regexp less to do

lib/model/content.dart

@@ -706,17 +706,15 @@ class UserMentionNode extends InlineContainerNode {
   const UserMentionNode({
     super.debugHtmlNode,
     required super.nodes,
-    // required this.mentionType,
-    // required this.isSilent,
   });
 
-  // We don't currently seem to need this information in code.  Instead,
+  // For the legacy design, we don't need this information in code; instead,
   // the inner text already shows how to communicate it to the user
   // (e.g., silent mentions' text lacks a leading "@"),
   // and we show that text in the same style for all types of @-mention.
-  // If we need this information in the future, go ahead and add it here.
-  //   final UserMentionType mentionType;
-  //   final bool isSilent;
+  // We'll need these for implementing the post-2023 Zulip design, though.
+  //   final UserMentionType mentionType; // TODO(#646)
+  //   final bool isSilent; // TODO(#647)
 }
 
 sealed class EmojiNode extends InlineContentNode {
@@ -872,6 +870,41 @@ class _ZulipContentParser {
     return descendant4.text.trim();
   }
 
+  UserMentionNode? parseUserMention(dom.Element element) {
+    assert(_debugParserContext == _ParserContext.inline);
+    assert(element.localName == 'span');
+    final debugHtmlNode = kDebugMode ? element : null;
+
+    final classes = element.className.split(' ')..sort();
+    assert(classes.contains('user-mention')
+        || classes.contains('user-group-mention'));
+    int i = 0;
+
+    if (i >= classes.length) return null;
+    if (classes[i] == 'silent') {
+      // A silent @-mention.  We ignore this flag; see [UserMentionNode].
+      i++;
+    }
+
+    if (i >= classes.length) return null;
+    if (classes[i] == 'user-mention' || classes[i] == 'user-group-mention') {
+      // The class we already knew we'd find before we called this function.
+      // We ignore the distinction between these; see [UserMentionNode].
+      i++;
+    }
+
+    if (i != classes.length) {
+      // There was some class we didn't expect.
+      return null;
+    }
+
+    // TODO assert UserMentionNode can't contain LinkNode;
+    //   either a debug-mode check, or perhaps we can make expectations much
+    //   tighter on a UserMentionNode's contents overall.
+    final nodes = parseInlineContentList(element.nodes);
+    return UserMentionNode(nodes: nodes, debugHtmlNode: debugHtmlNode);
+  }
+
   /// The links found so far in the current block inline container.
   ///
   /// Empty is represented as null.
@@ -884,12 +917,12 @@ class _ZulipContentParser {
     return result;
   }
 
-  static final _userMentionClassNameRegexp = () {
-    // This matches a class `user-mention` or `user-group-mention`,
-    // plus an optional class `silent`, appearing in either order.
-    const mentionClass = r"user(?:-group)?-mention";
-    return RegExp("^(?:$mentionClass(?: silent)?|silent $mentionClass)\$");
-  }();
+  /// Matches all className values that could be a UserMentionNode,
+  /// and no className values that could be any other type of node.
+  // Specifically, checks for `user-mention` or `user-group-mention`
+  // as a member of the list.
+  static final _userMentionClassNameRegexp = RegExp(
+    r"(^| )" r"user(?:-group)?-mention" r"( |$)");
 
   static final _emojiClassNameRegexp = () {
     const specificEmoji = r"emoji(?:-[0-9a-f]+)+";
@@ -944,10 +977,7 @@ class _ZulipContentParser {
 
     if (localName == 'span'
         && _userMentionClassNameRegexp.hasMatch(className)) {
-      // TODO assert UserMentionNode can't contain LinkNode;
-      //   either a debug-mode check, or perhaps we can make expectations much
-      //   tighter on a UserMentionNode's contents overall.
-      return UserMentionNode(nodes: nodes(), debugHtmlNode: debugHtmlNode);
+      return parseUserMention(element) ?? unimplemented();
     }
 
     if (localName == 'span'