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/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,