grep: emit GNU's 'Invalid range end' for reversed bracket ranges

A reversed range like [b-a] makes oniguruma fail with 'empty range in
char class', which uu_grep wrapped as 'invalid pattern "[b-a]": ...'.
GNU grep instead prints the bare POSIX diagnostic 'Invalid range end'
and exits 2. Translate the oniguruma error code
(ONIGERR_EMPTY_RANGE_IN_CHAR_CLASS) to GNU's wording, leaving other
compile errors to fall back to oniguruma's text. Fixes the GNU testsuite
'reversed-range-endpoints' test.

src/matcher.rs

@@ -8,7 +8,9 @@ use onig::{
     EncodedBytes, Regex, RegexOptions, Region, SearchOptions, Syntax, SyntaxBehavior,
     SyntaxOperator,
 };
-use onig_sys::{OnigEncCtype_ONIGENC_CTYPE_WORD, OnigEncodingUTF8};
+use onig_sys::{
+    ONIGERR_EMPTY_RANGE_IN_CHAR_CLASS, OnigEncCtype_ONIGENC_CTYPE_WORD, OnigEncodingUTF8,
+};
 use uucore::error::{UResult, USimpleError};
 
 pub struct Matcher<'a> {
@@ -240,7 +242,12 @@ impl CompiledPattern {
 
         fn compile_with(pattern: &str, syntax: &Syntax, options: RegexOptions) -> UResult<Regex> {
             Regex::with_options_and_encoding(pattern, options, syntax).map_err(|err| {
-                USimpleError::new(2, format!("invalid pattern \"{pattern}\": {err}"))
+                // Prefer GNU grep's wording for the errors it has a dedicated
+                // message for; fall back to oniguruma's text otherwise.
+                match gnu_error_message(err.code()) {
+                    Some(msg) => USimpleError::new(2, msg.to_string()),
+                    None => USimpleError::new(2, format!("invalid pattern \"{pattern}\": {err}")),
+                }
             })
         }
 
@@ -296,6 +303,19 @@ impl CompiledPattern {
     }
 }
 
+/// Map an oniguruma compile-error code to GNU grep's wording for the same
+/// condition, when one exists. GNU emits a bare POSIX-style diagnostic (e.g.
+/// `Invalid range end`) rather than oniguruma's phrasing, so translating keeps
+/// us byte-compatible. Returns `None` for errors with no GNU equivalent, where
+/// the caller falls back to oniguruma's own message.
+fn gnu_error_message(code: i32) -> Option<&'static str> {
+    match code {
+        // e.g. `[b-a]`: a range whose end precedes its start.
+        ONIGERR_EMPTY_RANGE_IN_CHAR_CLASS => Some("Invalid range end"),
+        _ => None,
+    }
+}
+
 /// Reject the confusing `[:name:]` bracket form the way GNU grep does.
 ///
 /// A bracket expression like `[:space:]` is almost always a misspelled

tests/test_grep.rs

@@ -180,6 +180,19 @@ fn lookalike_brackets_are_not_confusing() {
         .no_output();
 }
 
+#[test]
+fn reversed_range_uses_gnu_wording() {
+    // A range like `[b-a]` is an error; GNU prints the bare POSIX diagnostic
+    // "Invalid range end" (not oniguruma's phrasing) and exits 2.
+    for args in [&["[b-a]"][..], &["-E", "[b-a]"][..]] {
+        let (_s, mut c) = ucmd();
+        c.args(args)
+            .pipe_in("x\n")
+            .fails_with_code(2)
+            .stderr_is("grep: Invalid range end\n");
+    }
+}
+
 #[test]
 fn fixed_string_is_literal() {
     // Metacharacters are not interpreted.