split: slash separator

src/uu/split/src/filenames.rs

@@ -230,7 +230,7 @@ impl Suffix {
             .get_one::<String>(OPT_ADDITIONAL_SUFFIX)
             .unwrap()
             .to_string();
-        if additional.contains('/') {
+        if additional.contains('/') || additional.contains('\\') {
             return Err(SuffixError::ContainsSeparator(additional));
         }
 

tests/by-util/test_split.rs

@@ -252,6 +252,10 @@ fn test_additional_suffix_no_slash() {
         .args(&["--additional-suffix", "a/b"])
         .fails()
         .usage_error("invalid suffix 'a/b', contains directory separator");
+    new_ucmd!()
+        .args(&["--additional-suffix", "a\\b"])
+        .fails()
+        .usage_error("invalid suffix 'a\\b', contains directory separator");
 }
 
 #[test]