Reported at https://crbug.com/gn/359

--- build/gen.py
+++ build/gen.py
@@ -143,7 +143,7 @@
   describe_output = subprocess.check_output(
       ['git', 'describe', 'HEAD', '--match', ROOT_TAG], shell=host.is_windows(),
       cwd=REPO_ROOT)
-  mo = re.match(ROOT_TAG + '-(\d+)-g([0-9a-f]+)', describe_output.decode())
+  mo = re.match(ROOT_TAG + r'-(\d+)-g([0-9a-f]+)', describe_output.decode())
   if not mo:
     raise ValueError(
         'Unexpected output from git describe when generating version header')