tac: add regex flavor translation for compatibility and new test case

src/uu/tac/src/tac.rs

@@ -223,11 +223,19 @@ fn buffer_tac(data: &[u8], before: bool, separator: &str) -> std::io::Result<()>
     Ok(())
 }
 
+// Make the regex flavor compatible with `regex` crate
+fn translate_regex_flavor(regex: &str) -> String {
+    let result: String = regex.replace("\\|", "|");
+    // TODO: are more translations needed?
+
+    result
+}
+
 #[allow(clippy::cognitive_complexity)]
 fn tac(filenames: &[OsString], before: bool, regex: bool, separator: &str) -> UResult<()> {
     // Compile the regular expression pattern if it is provided.
     let maybe_pattern = if regex {
-        match regex::bytes::Regex::new(separator) {
+        match regex::bytes::Regex::new(&translate_regex_flavor(separator)) {
             Ok(p) => Some(p),
             Err(e) => return Err(TacError::InvalidRegex(e).into()),
         }

tests/by-util/test_tac.rs

@@ -347,3 +347,12 @@ fn test_stdin_bad_tmpdir_fallback() {
         .succeeds()
         .stdout_is("c\nb\na\n");
 }
+
+#[test]
+fn test_regex_or_operator() {
+    new_ucmd!()
+        .args(&["-r", "-s", r"[^x]\|x"])
+        .pipe_in("abc")
+        .succeeds()
+        .stdout_contains("cba");
+}