regex: fix matching bug when text anchors are used

It turns out that if there are text anchors (that is, \A or \z, or ^/$
when multi-line is disabled), then the "fast" line searching path isn't
quite correct. Since searching without multi-line mode is exceptionally
rare, we just look for the presence of text anchors and specifically
disable the line terminator option in 'grep-regex'. This in turn
inhibits the "fast" line searching path.

Fixes #2260

crates/regex/src/config.rs

@@ -175,6 +175,36 @@ impl ConfiguredHIR {
         self.config.crlf && self.expr.is_line_anchored_end()
     }
 
+    /// Returns the line terminator configured on this expression.
+    ///
+    /// When we have beginning/end anchors (NOT line anchors), the fast line
+    /// searching path isn't quite correct. Or at least, doesn't match the
+    /// slow path. Namely, the slow path strips line terminators while the
+    /// fast path does not. Since '$' (when multi-line mode is disabled)
+    /// doesn't match at line boundaries, the existence of a line terminator
+    /// might cause it to not match when it otherwise would with the line
+    /// terminator stripped.
+    ///
+    /// Since searching with text anchors is exceptionally rare in the
+    /// context of line oriented searching (multi-line mode is basically
+    /// always enabled), we just disable this optimization when there are
+    /// text anchors. We disable it by not returning a line terminator, since
+    /// without a line terminator, the fast search path can't be executed.
+    ///
+    /// See: https://github.com/BurntSushi/ripgrep/issues/2260
+    pub fn line_terminator(&self) -> Option<LineTerminator> {
+        if self.is_any_anchored() {
+            None
+        } else {
+            self.config.line_terminator
+        }
+    }
+
+    /// Returns true if and only if the underlying HIR has any text anchors.
+    fn is_any_anchored(&self) -> bool {
+        self.expr.is_any_anchored_start() || self.expr.is_any_anchored_end()
+    }
+
     /// Builds a regular expression from this HIR expression.
     pub fn regex(&self) -> Result<Regex, Error> {
         self.pattern_to_regex(&self.expr.to_string())

crates/regex/src/matcher.rs

@@ -52,8 +52,12 @@ impl RegexMatcherBuilder {
 
         let matcher = RegexMatcherImpl::new(&chir)?;
         log::trace!("final regex: {:?}", matcher.regex());
+        let mut config = self.config.clone();
+        // We override the line terminator in case the configured expr doesn't
+        // support it.
+        config.line_terminator = chir.line_terminator();
         Ok(RegexMatcher {
-            config: self.config.clone(),
+            config,
             matcher,
             fast_line_regex,
             non_matching_bytes,

crates/searcher/src/searcher/core.rs

@@ -108,7 +108,7 @@ impl<'s, M: Matcher, S: Sink> Core<'s, M, S> {
     }
 
     pub fn match_by_line(&mut self, buf: &[u8]) -> Result<bool, S::Error> {
-        if self.is_line_by_line_fast() {
+        if dbg!(self.is_line_by_line_fast()) {
             self.match_by_line_fast(buf)
         } else {
             self.match_by_line_slow(buf)

crates/searcher/src/searcher/glue.rs

@@ -1512,4 +1512,31 @@ and exhibited clearly, with a label attached.\
             )
             .unwrap();
     }
+
+    // See: https://github.com/BurntSushi/ripgrep/issues/2260
+    #[test]
+    fn regression_2260() {
+        use grep_regex::RegexMatcherBuilder;
+
+        use crate::SearcherBuilder;
+
+        let matcher = RegexMatcherBuilder::new()
+            .line_terminator(Some(b'\n'))
+            .build(r"^\w+$")
+            .unwrap();
+        let mut searcher = SearcherBuilder::new().line_number(true).build();
+
+        let mut matched = false;
+        searcher
+            .search_slice(
+                &matcher,
+                b"GATC\n",
+                crate::sinks::UTF8(|_, _| {
+                    matched = true;
+                    Ok(true)
+                }),
+            )
+            .unwrap();
+        assert!(matched);
+    }
 }