ripgrep: add --auto-hybrid-regex flag

This flag, when set, will automatically dispatch to PCRE2 if the given
regex cannot be compiled by Rust's regex engine. If both engines fail to
compile the regex, then both errors are surfaced.

Closes #1155

tests/feature.rs

@@ -681,6 +681,21 @@ rgtest!(f1138_no_ignore_dot, |dir: Dir, mut cmd: TestCommand| {
     eqnice!("bar\n", cmd.arg("--ignore-file").arg(".fzf-ignore").stdout());
 });
 
+// See: https://github.com/BurntSushi/ripgrep/issues/1155
+rgtest!(f1155_auto_hybrid_regex, |dir: Dir, mut cmd: TestCommand| {
+    // No sense in testing a hybrid regex engine with only one engine!
+    if !dir.is_pcre2() {
+        return;
+    }
+
+    dir.create("sherlock", SHERLOCK);
+    cmd.arg("--no-pcre2").arg("--auto-hybrid-regex").arg(r"(?<=the )Sherlock");
+
+    let expected = "\
+sherlock:For the Doctor Watsons of this world, as opposed to the Sherlock
+";
+    eqnice!(expected, cmd.stdout());
+});
 
 // See: https://github.com/BurntSushi/ripgrep/issues/1207
 //