Only allow 1. to interrupt paragraphs in significantNewlines mode

src/Parser/BlockParser.php

@@ -2533,9 +2533,10 @@ protected function startsNewBlockSignificant(string $line): bool
                 // Fenced divs: :{3,}
                 return isset($line[1], $line[2]) && $line[1] === ':' && $line[2] === ':';
             default:
-                // Ordered lists: digit or letter followed by . or )
-                if (ctype_digit($first) || ctype_alpha($first)) {
-                    return preg_match('/^(\d+|[a-zA-Z])[.)]\s/', $line) === 1;
+                // Only 1. or 1) can interrupt paragraphs (CommonMark rule)
+                // Prevents "1985. That year..." from becoming a list
+                if ($first === '1') {
+                    return preg_match('/^1[.)]\s/', $line) === 1;
                 }
 
                 return false;

tests/TestCase/SignificantNewlinesTest.php

@@ -291,4 +291,52 @@ public function testHeadingInterruptsParagraphInSignificantNewlinesMode(): void
         $this->assertInstanceOf(Paragraph::class, $children[0]);
         $this->assertInstanceOf(Heading::class, $children[1]);
     }
+
+    // ==================== CommonMark Ordered List Rule ====================
+
+    public function testOnlyOneCanInterruptParagraph(): void
+    {
+        // Only "1." can interrupt a paragraph (CommonMark rule)
+        $parser = new BlockParser(significantNewlines: true);
+        $doc = $parser->parse("Steps:\n1. First step");
+
+        $children = $doc->getChildren();
+        $this->assertCount(2, $children);
+        $this->assertInstanceOf(Paragraph::class, $children[0]);
+        $this->assertInstanceOf(ListBlock::class, $children[1]);
+    }
+
+    public function testYearDoesNotBecomeList(): void
+    {
+        // "1985." should NOT interrupt - prevents years becoming lists
+        $parser = new BlockParser(significantNewlines: true);
+        $doc = $parser->parse("My favorite year was\n1985. It was great.");
+
+        $children = $doc->getChildren();
+        $this->assertCount(1, $children);
+        $this->assertInstanceOf(Paragraph::class, $children[0]);
+    }
+
+    public function testHighNumberedListDoesNotInterrupt(): void
+    {
+        // "5." should NOT interrupt paragraphs
+        $parser = new BlockParser(significantNewlines: true);
+        $doc = $parser->parse("Continue from step\n5. Do this thing");
+
+        $children = $doc->getChildren();
+        $this->assertCount(1, $children);
+        $this->assertInstanceOf(Paragraph::class, $children[0]);
+    }
+
+    public function testHighNumberedListAfterBlankLine(): void
+    {
+        // With blank line, any number can start a list
+        $parser = new BlockParser(significantNewlines: true);
+        $doc = $parser->parse("Continue from step\n\n5. Do this thing");
+
+        $children = $doc->getChildren();
+        $this->assertCount(2, $children);
+        $this->assertInstanceOf(Paragraph::class, $children[0]);
+        $this->assertInstanceOf(ListBlock::class, $children[1]);
+    }
 }