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

src/args.rs

@@ -599,6 +599,25 @@ impl ArgMatches {
         if self.is_present("pcre2") {
             let matcher = self.matcher_pcre2(patterns)?;
             Ok(PatternMatcher::PCRE2(matcher))
+        } else if self.is_present("auto-hybrid-regex") {
+            let rust_err = match self.matcher_rust(patterns) {
+                Ok(matcher) => return Ok(PatternMatcher::RustRegex(matcher)),
+                Err(err) => err,
+            };
+            log::debug!(
+                "error building Rust regex in hybrid mode:\n{}", rust_err,
+            );
+            let pcre_err = match self.matcher_pcre2(patterns) {
+                Ok(matcher) => return Ok(PatternMatcher::PCRE2(matcher)),
+                Err(err) => err,
+            };
+            Err(From::from(format!(
+                "regex could not be compiled with either the default regex \
+                 engine or with PCRE2.\n\n\
+                 default regex engine error:\n{}\n{}\n{}\n\n\
+                 PCRE2 regex engine error:\n{}",
+                 "~".repeat(79), rust_err, "~".repeat(79), pcre_err,
+            )))
         } else {
             let matcher = match self.matcher_rust(patterns) {
                 Ok(matcher) => matcher,