Avoid bounded backtracker when max haystack length is zero

regex-automata/src/meta/wrappers.rs

@@ -177,7 +177,8 @@ impl BoundedBacktracker {
         }
         // If the backtracker is just going to return an error because the
         // haystack is too long, then obviously do not use it.
-        if input.get_span().len() > engine.max_haystack_len() {
+        let max_haystack_len = engine.max_haystack_len();
+        if max_haystack_len == 0 || input.get_span().len() > max_haystack_len {
             return None;
         }
         Some(engine)

regex-automata/tests/meta/suite.rs

@@ -39,6 +39,14 @@ fn default() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn backtracker_zero_max_haystack_len() -> Result<()> {
+    let config = Regex::config().nfa_size_limit(Some(1_000_000_000));
+    let re = Regex::builder().configure(config).build(r"^.{0,404600}$")?;
+    assert!(re.is_match(""));
+    Ok(())
+}
+
 /// Tests the default configuration minus the full DFA.
 #[test]
 fn no_dfa() -> Result<()> {