grep: honor GNU buffer anchors (#56)

src/matcher.rs

@@ -269,6 +269,10 @@ impl CompiledPattern {
             // GNU grep supports `{,n}` as an alias for `{0,n}`.
             syntax.enable_behavior(SyntaxBehavior::SYNTAX_BEHAVIOR_ALLOW_INTERVAL_LOW_ABBREV);
         }
+        if matches!(config.regex_mode, RegexMode::Basic | RegexMode::Extended) {
+            // GNU grep supports \` and \' as buffer anchors in BRE and ERE.
+            syntax.enable_operators(SyntaxOperator::SYNTAX_OPERATOR_ESC_GNU_BUF_ANCHOR);
+        }
         if config.regex_mode == RegexMode::Perl {
             // GNU grep supports `(?P<name>...)`.
             // Unfortunately, the onig crate defines the OP2 flag without the

tests/test_grep.rs

@@ -85,6 +85,21 @@ fn bre_gnu_extensions() {
         .stdout_only("*foo\n**foo\n");
 }
 
+#[test]
+fn gnu_buffer_anchors() {
+    let (_s, mut c) = ucmd();
+    c.args(&[r"\`c\|r\'"])
+        .pipe_in("cat\nscat\ntar\ndog\n")
+        .succeeds()
+        .stdout_only("cat\ntar\n");
+
+    let (_s, mut c) = ucmd();
+    c.args(&["-E", r"\`c|r\'"])
+        .pipe_in("cat\nscat\ntar\ndog\n")
+        .succeeds()
+        .stdout_only("cat\ntar\n");
+}
+
 #[test]
 fn ere_metacharacters() {
     let cases: &[(&[&str], &str, &str)] = &[