LibMarkdown: Make thematic break parsing more correct

also fix a conflict with lists and thematic breaks

Userland/Libraries/LibMarkdown/ContainerBlock.cpp

@@ -108,11 +108,11 @@ OwnPtr<ContainerBlock> ContainerBlock::parse(LineIterator& lines)
         }
 
         bool any = try_parse_block<Table>(lines, blocks)
+            || try_parse_block<HorizontalRule>(lines, blocks)
             || try_parse_block<List>(lines, blocks)
             || try_parse_block<CodeBlock>(lines, blocks)
             || try_parse_block<CommentBlock>(lines, blocks)
             || try_parse_block<Heading>(lines, blocks)
-            || try_parse_block<HorizontalRule>(lines, blocks)
             || try_parse_block<BlockQuote>(lines, blocks);
 
         if (any) {

Userland/Libraries/LibMarkdown/HorizontalRule.cpp

@@ -8,6 +8,7 @@
 #include <AK/StringBuilder.h>
 #include <LibMarkdown/HorizontalRule.h>
 #include <LibMarkdown/Visitor.h>
+#include <LibRegex/Regex.h>
 
 namespace Markdown {
 
@@ -34,6 +35,8 @@ RecursionDecision HorizontalRule::walk(Visitor& visitor) const
     return RecursionDecision::Continue;
 }
 
+static Regex<ECMA262> thematic_break_re("^ {0,3}([\\*\\-_])(\\s*\\1\\s*){2,}$");
+
 OwnPtr<HorizontalRule> HorizontalRule::parse(LineIterator& lines)
 {
     if (lines.is_end())
@@ -41,17 +44,10 @@ OwnPtr<HorizontalRule> HorizontalRule::parse(LineIterator& lines)
 
     StringView line = *lines;
 
-    if (line.length() < 3)
-        return {};
-    if (!line.starts_with('-') && !line.starts_with('_') && !line.starts_with('*'))
+    auto match = thematic_break_re.match(line);
+    if (!match.success)
         return {};
 
-    auto first_character = line.characters_without_null_termination()[0];
-    for (auto ch : line) {
-        if (ch != first_character)
-            return {};
-    }
-
     ++lines;
     return make<HorizontalRule>();
 }