Fix backslash replacement issue

cibuildwheel/util.py

@@ -76,7 +76,13 @@ def format_safe(template: str, **kwargs: Any) -> str:
             re.VERBOSE,
         )
 
-        result = re.sub(find_pattern, str(value), result)
+        # we use a lambda for repl to prevent re.sub interpreting backslashes
+        # in repl as escape sequences
+        result = re.sub(
+            pattern=find_pattern,
+            repl=lambda _: str(value),
+            string=result,
+        )
 
         # transform escaped sequences into their literal equivalents
         result = result.replace(f"\\{{{key}}}", f"{{{key}}}")

unit_test/utils_test.py

@@ -29,6 +29,14 @@ def test_prepare_command():
         == "python -m {something.abc[4]:3f}"
     )
 
+    # test backslashes in the replacement
+    assert (
+        prepare_command(
+            "command {wheel} \\Users\\Temp\\output_dir", wheel="\\Temporary Files\\cibw"
+        )
+        == "command \\Temporary Files\\cibw \\Users\\Temp\\output_dir"
+    )
+
     # test some unusual syntax that used to trip up the str.format approach
     assert (
         prepare_command("{a}{a,b}{b:.2e}{c}{d%s}{e:3}{f[0]}", a="42", b="3.14159")