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/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);
+    }
 }