diff --git a/PFERD/transform.py b/PFERD/transform.py
index 16769df..7a05dd1 100644
--- a/PFERD/transform.py
+++ b/PFERD/transform.py
@@ -5,6 +5,8 @@ only files whose names match a regex, or renaming files from one numbering
 scheme to another.
 """
 
+import os
+import re
 from dataclasses import dataclass
 from pathlib import PurePath
 from typing import Callable, List, Optional, TypeVar
@@ -45,7 +47,8 @@ def apply_transform(
 
 # Transform combinators
 
-keep = lambda path: path
+def keep(path: PurePath) -> Optional[PurePath]:
+    return path
 
 def attempt(*args: Transform) -> Transform:
     def inner(path: PurePath) -> Optional[PurePath]:
@@ -125,3 +128,15 @@ def re_rename(regex: Regex, target: str) -> Transform:
             return path.with_name(target.format(*groups))
         return None
     return inner
+
+
+def sanitize_windows_path(path: PurePath) -> Optional[PurePath]:
+    """
+    A small function to escape characters that are forbidden in windows path names.
+    This method is a no-op on other operating systems.
+    """
+    # Escape windows illegal path characters
+    if os.name == 'nt':
+        sanitized_parts = [re.sub(r'[<>:"/|?]', "_", x) for x in list(path.parts)]
+        return PurePath(*sanitized_parts)
+    return path
diff --git a/sync_url.py b/sync_url.py
index 2b8bc56..ddd239a 100755
--- a/sync_url.py
+++ b/sync_url.py
@@ -5,27 +5,17 @@ A simple script to download a course by name from ILIAS.
 """
 
 import argparse
-import os
-import re
-from pathlib import Path, PurePath
-from typing import Optional
+from pathlib import Path
 from urllib.parse import urlparse
 
 from PFERD import Pferd
 from PFERD.cookie_jar import CookieJar
 from PFERD.ilias import (IliasCrawler, IliasElementType,
                          KitShibbolethAuthenticator)
+from PFERD.transform import sanitize_windows_path
 from PFERD.utils import to_path
 
 
-def sanitize_path(path: PurePath) -> Optional[PurePath]:
-    # Escape windows illegal path characters
-    if os.name == 'nt':
-        sanitized_parts = [re.sub(r'[<>:"/|?]', "_", x) for x in list(path.parts)]
-        return PurePath(*sanitized_parts)
-    return path
-
-
 def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument("--test-run", action="store_true")
@@ -71,7 +61,7 @@ def main() -> None:
         full_url=args.url,
         cookies=args.cookies,
         dir_filter=dir_filter,
-        transform=sanitize_path
+        transform=sanitize_windows_path
     )