Upgrade rust/crates/regex to 1.3.9

Test: None
Change-Id: Ic5726cda9566f808b7dd21d8ce9a3045250f8a54
diff --git a/src/lib.rs b/src/lib.rs
index 2a74bf8..e0a0975 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -152,8 +152,9 @@
 ```
 
 If you wish to match against whitespace in this mode, you can still use `\s`,
-`\n`, `\t`, etc. For escaping a single space character, you can use its hex
-character code `\x20` or temporarily disable the `x` flag, e.g., `(?-x: )`.
+`\n`, `\t`, etc. For escaping a single space character, you can escape it
+directly with `\ `, use its hex character code `\x20` or temporarily disable
+the `x` flag, e.g., `(?-x: )`.
 
 # Example: match multiple regular expressions simultaneously
 
@@ -621,8 +622,8 @@
 
 #[cfg(feature = "perf-literal")]
 extern crate aho_corasick;
-#[cfg(test)]
-extern crate doc_comment;
+// #[cfg(doctest)]
+// extern crate doc_comment;
 #[cfg(feature = "perf-literal")]
 extern crate memchr;
 #[cfg(test)]
@@ -632,8 +633,8 @@
 #[cfg(feature = "perf-cache")]
 extern crate thread_local;
 
-#[cfg(test)]
-doc_comment::doctest!("../README.md");
+// #[cfg(doctest)]
+// doc_comment::doctest!("../README.md");
 
 #[cfg(feature = "std")]
 pub use error::Error;