diff --git a/src/matcher.rs b/src/matcher.rs
index adda24a..324e75e 100644
--- a/src/matcher.rs
+++ b/src/matcher.rs
@@ -9,7 +9,7 @@ use onig::{
     SyntaxBehavior, SyntaxOperator,
 };
 use onig_sys::{
-    ONIGERR_EMPTY_RANGE_IN_CHAR_CLASS, ONIGERR_RETRY_LIMIT_IN_MATCH_OVER,
+    ONIGERR_EMPTY_RANGE_IN_CHAR_CLASS, ONIGERR_INVALID_BACKREF, ONIGERR_RETRY_LIMIT_IN_MATCH_OVER,
     ONIGERR_RETRY_LIMIT_IN_SEARCH_OVER, OnigEncCtype_ONIGENC_CTYPE_WORD, OnigEncodingUTF8,
 };
 use std::io;
@@ -248,22 +248,29 @@ impl CompiledPattern {
             options |= RegexOptions::REGEX_OPTION_IGNORECASE;
         }
 
-        fn compile_with(pattern: &str, syntax: &Syntax, options: RegexOptions) -> UResult<Regex> {
+        let mode = config.regex_mode;
+        fn compile_with(
+            pattern: &str,
+            syntax: &Syntax,
+            options: RegexOptions,
+            mode: RegexMode,
+        ) -> UResult<Regex> {
             Regex::with_options_and_encoding(pattern, options, syntax).map_err(|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()) {
+                match gnu_error_message(err.code(), mode) {
                     Some(msg) => USimpleError::new(2, msg.to_string()),
                     None => USimpleError::new(2, format!("invalid pattern \"{pattern}\": {err}")),
                 }
             })
         }
 
-        let leftmost = compile_with(pattern, &syntax, options)?;
+        let leftmost = compile_with(pattern, &syntax, options, mode)?;
         let longest_anchored = compile_with(
             pattern,
             &syntax,
             options | RegexOptions::REGEX_OPTION_FIND_LONGEST,
+            mode,
         )?;
         Ok(Self {
             leftmost,
@@ -336,10 +343,16 @@ fn match_error(err: Error) -> io::Error {
 /// `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> {
+fn gnu_error_message(code: i32, mode: RegexMode) -> 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"),
+        // e.g. `(.)\2`: a back-reference to a group that does not exist. GNU
+        // (via PCRE2) and gnulib's regex word this differently.
+        ONIGERR_INVALID_BACKREF if mode == RegexMode::Perl => {
+            Some("reference to non-existent subpattern")
+        }
+        ONIGERR_INVALID_BACKREF => Some("Invalid back reference"),
         _ => None,
     }
 }
diff --git a/tests/test_grep.rs b/tests/test_grep.rs
index bfe458b..d3d5d77 100644
--- a/tests/test_grep.rs
+++ b/tests/test_grep.rs
@@ -130,10 +130,11 @@ fn ere_invalid_pattern_is_error() {
 fn confusing_bracket_class_is_error() {
     // GNU grep rejects the misspelled `[:name:]` form (meant to be
     // `[[:name:]]`) with a dedicated diagnostic and exit code 2.
+    // No piped input: the pattern is rejected at compile time, before stdin is
+    // read, so feeding stdin would race with the child exiting (broken pipe).
     for pattern in ["[:space:]", "[:digit:]", "[^:space:]", "x[:space:]y"] {
         let (_s, mut c) = ucmd();
         c.args(&[pattern])
-            .pipe_in("x\n")
             .fails_with_code(2)
             .stderr_is("grep: character class syntax is [[:space:]], not [:space:]\n");
     }
@@ -141,7 +142,6 @@ fn confusing_bracket_class_is_error() {
     // The same diagnostic applies in extended mode.
     let (_s, mut c) = ucmd();
     c.args(&["-E", "[:space:]"])
-        .pipe_in("x\n")
         .fails_with_code(2)
         .stderr_is("grep: character class syntax is [[:space:]], not [:space:]\n");
 }
@@ -184,10 +184,10 @@ fn lookalike_brackets_are_not_confusing() {
 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.
+    // No piped input: the pattern is rejected before stdin is read.
     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");
     }
@@ -205,6 +205,33 @@ fn pcre_backtracking_limit_does_not_abort() {
         .stderr_contains("backtracking limit");
 }
 
+#[test]
+fn invalid_backreference_uses_gnu_wording() {
+    // A back-reference to a non-existent group is worded differently by GNU
+    // depending on the engine: PCRE (-P) vs gnulib regex (BRE/ERE).
+    // No piped input: these patterns are rejected before stdin is read.
+    let (_s, mut c) = ucmd();
+    c.args(&["-P", r"(.)\2"])
+        .fails_with_code(2)
+        .stderr_is("grep: reference to non-existent subpattern\n");
+
+    for args in [&["-E", r"(.)\2"][..], &[r"\(.\)\2"][..]] {
+        let (_s, mut c) = ucmd();
+        c.args(args)
+            .fails_with_code(2)
+            .stderr_is("grep: Invalid back reference\n");
+    }
+
+    // A valid back-reference with -Pw / -Px must still match.
+    for flag in ["-Pw", "-Px"] {
+        let (_s, mut c) = ucmd();
+        c.args(&[flag, r"(.)\1"])
+            .pipe_in("aa\n")
+            .succeeds()
+            .stdout_is("aa\n");
+    }
+}
+
 #[test]
 fn fixed_string_is_literal() {
     // Metacharacters are not interpreted.