fix: parse setext headings

src/Rules/ParagraphRule.php

@@ -5,6 +5,7 @@
 use Tempest\Markdown\Parser;
 use Tempest\Markdown\Rule;
 use Tempest\Markdown\Token;
+use Tempest\Markdown\Tokens\HeadingToken;
 use Tempest\Markdown\Tokens\ParagraphToken;
 
 final readonly class ParagraphRule implements Rule
@@ -34,6 +35,14 @@ public function parse(Parser $parser): Token
             $content .= $parser->consumeWhile(Parser::NEW_LINE);
         }
 
+        $trimmed = rtrim($content, "\r\n");
+        if (preg_match('/\A(.+)\r?\n {0,3}(=+|-+)[ \t]*\z/s', $trimmed, $matches)) {
+            $heading = trim($matches[1]);
+            $id = mb_strtolower($heading) |> (fn (string $value) => trim(preg_replace('/[^\p{L}\p{N}]+/u', '-', $value) ?? '', '-'));
+
+            return new HeadingToken($heading, $matches[2][0] === '=' ? 1 : 2, $id);
+        }
+
         return new ParagraphToken($content);
     }
 }

tests/Rules/ParagraphRuleTest.php

@@ -41,4 +41,21 @@ public function test_paragraph_without_trailing_newline(): void
 
         $this->assertSame('<p>Hello, world!</p>', $html);
     }
+
+    #[Test]
+    public function test_setext_headings(): void
+    {
+        $parser = new Parser(highlighter: null, rules: [new ParagraphRule()]);
+
+        $this->assertSame('<h1 id="first-heading">First heading</h1>', (string) $parser->parse("First heading\n===="));
+        $this->assertSame('<h2 id="second-heading">Second heading</h2>', (string) $parser->parse("Second heading\n-----\n"));
+    }
+
+    #[Test]
+    public function test_setext_heading_can_span_multiple_lines(): void
+    {
+        $parser = new Parser(highlighter: null, rules: [new ParagraphRule()]);
+
+        $this->assertSame("<h2 id=\"first-second\">First\nSecond</h2>", (string) $parser->parse("First\nSecond\n---"));
+    }
 }