fix: prevent infinite loop caused by unnormalized line breaks

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 - Fix a potential security issue in the URL escaping logic of the Markdown renderer.
 - Fix GFM table indentation in list.
+- Fix an infinite loop caused by unnormalized line breaks.
 
 ## v2.2.2(2026-01-05)
 

marko/source.py

@@ -15,7 +15,8 @@
 
 
 def _preprocess_text(text: str) -> str:
-    return text.replace("\r\n", "\n")
+    # Normalize line terminators so block parsers can always advance on line reads.
+    return text.replace("\r\n", "\n").replace("\r", "\n").replace("\f", "\n")
 
 
 class Source:
@@ -102,7 +103,8 @@ def expect_re(self, regexp: Pattern[str] | str) -> Match[str] | None:
         :returns: the match object.
         """
         prefix_len = self.match_prefix(
-            self.prefix, self.next_line(require_prefix=False)  # type: ignore
+            self.prefix,
+            self.next_line(require_prefix=False),  # type: ignore
         )
         if prefix_len >= 0:
             match = self._expect_re(regexp, self.pos + prefix_len)

tests/test_basic.py

@@ -2,17 +2,21 @@
 import re
 import textwrap
 
-from marko.source import Source
 import pytest
 
 import marko
 from marko import block
 from marko.ast_renderer import ASTRenderer, XMLRenderer
 from marko.md_renderer import MarkdownRenderer
+from marko.source import Source
 from tests.normalize import normalize_html
 
 
 class TestBasic:
+    @pytest.mark.parametrize("text", ["-\r-", "-\f-"])
+    def test_non_lf_line_terminators_no_infinite_loop(self, text):
+        assert marko.convert(text) == marko.convert("-\n-")
+
     def test_xml_renderer(self):
         text = "[Overview](#overview)\n\n* * *"
         markdown = marko.Markdown(renderer=XMLRenderer)
@@ -169,7 +173,6 @@ class CustomElement(block.BlockElement):
             def __init__(self, match: re.Match[str]) -> None:
                 self.inline_body = match.group(1).strip()
 
-
             @classmethod
             def match(cls, source: Source) -> "re.Match[str] | None":
                 return source.expect_re(cls.pattern)